Add initial stub for WCFS program and tests.
WCFS functionality will be added step-by-step in follow-up commits.

Some preliminary history:

0ae88a32       X .nxdtest: Verify Go bits with GOMAXPROCS=1,2,`nproc`
23528eb4       X wcfs: make it to use go modules for dependencies

Upcoming libwcfs (C++ part of WCFS client) will need to use virtmem code
and link to libvirtmem.

Do what we can do without gdb and then tail to regular segmentation
fault. With core file gdb can still be used, but it is handy if we
already can get traceback of the crash into the log automatically.

TODO better use https://github.com/ianlancetaylor/libbacktrace because
backtrace_symbols often does not provide symbolic information.

We do not do this now because libbacktrace is not always automatically
installed.

This makes sure that those programs are always built afresh instead
being stuck at outdated build. This is needed because corresponding test .c
file includes many other .c files and we don't implement dependency tracking.

For failing case compiler-rt support was added in 2014 - 5 years ago
(see links in removed code).

So that it can be available to everyone and in particular B & friends to
be available from introduced importable golang.testing package. The move
itself:

	pygolang@9bf03d9c

While moving the code was restructured / improved a bit and py.bench
interface reworked to mimic `go test -bench` in defaults.

Benchmark the time it takes for virtmem to handle pagefault with noop
loadblk for loadblk both implemented in C and in Python.

On my computer it is:

	name          µs/op
	PagefaultC    269 ± 0%
	pagefault_py  291 ± 0%

Quite a big time in other words.

It turned out to be mostly spent in fallocate'ing pages on tmpfs from
/dev/shm. Part of the above 269 µs/op is taken by freeing (reclaiming)
pages back when benchmarking work size exceed /dev/shm size, and part to
allocating.

If I limit the work size (via npage in benchmem.c) to be less than whole
/dev/shm it starts to be ~ 170 µs/op and with additional tracing it
shows as something like this:

    	.. on_pagefault_start   0.954 µs
    	.. vma_on_pagefault_pre 0.954 µs
    	.. ramh_alloc_page_pre  0.954 µs
    	.. ramh_alloc_page      169.992 µs
    	.. vma_on_pagefault     172.853 µs
    	.. vma_on_pagefault_pre 172.853 µs
    	.. vma_on_pagefault     174.046 µs
    	.. on_pagefault_end     174.046 µs
    	.. whole:               171.900 µs

so almost all time is spent in ramh_alloc_page which is doing the fallocate:

	https://lab.nexedi.com/nexedi/wendelin.core/blob/f11386a4/bigfile/ram_shmfs.c#L125

Simple benchmark[1] confirmed it is indeed the case for fallocate(tmpfs) to be
relatively slow[2] (and that for recent kernels it regressed somewhat
compared to Linux 3.16). Profile flamegraph for that benchmark[3] shows
internal loading of shmem_fallocate which for 1 hardware page is not
that too slow (e.g. <1µs) but when a request comes for a region
internally performs it page by page and so accumulates that ~ 170µs for 2M.

I've tried to briefly rerun the benchmark with huge pages activated on /dev/shm via

	mount /dev/shm -o huge=always,remount

as both regular user and as root but it was executing several times
slower. Probably something to investigate more later.

[1] https://lab.nexedi.com/kirr/misc/blob/4f84a06e/tmpfs/t_fallocate.c
[2] https://lab.nexedi.com/kirr/misc/blob/4f84a06e/tmpfs/1.txt
[3] https://lab.nexedi.com/kirr/misc/raw/4f84a06e/tmpfs/fallocate-2M-nohuge.svg

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At present several threads running can corrupt internal virtmem
datastructures (e.g. ram->lru_list, fileh->pagemap, etc).

This can happen even if we have zope instances only with 1 worker thread
- because there are other "system" thread, and python garbage collection
can trigger at any thread, so if a virtmem object, e.g. VMA or FileH was
there sitting at GC queue to be collected, their collection, and thus
e.g. vma_unmap() and fileh_close() will be called from
different-from-worker thread.

Because of that virtmem just has to be aware of threads not to allow
internal datastructure corruption.

On the other hand, the idea of introducing userspace virtual memory
manager turned out to be not so good from performance and complexity
point of view, and thus the plan is to try to move it back into the
kernel. This way it does not make sense to do a well-optimised locking
implementation for userspace version.

So we do just a simple single "protect-all" big lock for virtmem.

Of a particular note is interaction with Python's GIL - any long-lived
lock has to be taken with GIL released, because else it can deadlock:

    t1  t2

    G
    V   G
   !G   V
    G

so we introduce helpers to make sure the GIL is not taken, and to retake
it back if we were holding it initially.

Those helpers (py_gil_ensure_unlocked / py_gil_retake_if_waslocked) are
symmetrical opposites to what Python provides to make sure the GIL is
locked (via PyGILState_Ensure / PyGILState_Release).

Otherwise, the patch is more-or-less straightforward application for
one-big-lock to protect everything idea.

Else, in a clean repo the tests do not build from scratch:

    $ make test
    x86_64-linux-gnu-gcc -pthread -g -Wall -D_GNU_SOURCE -std=gnu99 -fplan9-extensions -Wno-declaration-after-statement -Wno-error=declaration-after-statement  -Iinclude -I3rdparty/ccan -I3rdparty/include   bigfile/tests/test_virtmem.c lib/bug.c lib/utils.c 3rdparty/ccan/ccan/tap/tap.c  -o bigfile/tests/test_virtmem.t
    In file included from include/wendelin/list.h:11:0,
                     from include/wendelin/bigfile/virtmem.h:33,
                     from bigfile/tests/../virtmem.c:23,
                     from bigfile/tests/test_virtmem.c:21:
    3rdparty/ccan/ccan/array_size/array_size.h:4:20: fatal error: config.h: No such file or directory
     #include "config.h"
                        ^
    compilation terminated.
    3rdparty/ccan/ccan/tap/tap.c:26:20: fatal error: config.h: No such file or directory
     #include "config.h"
                        ^
    compilation terminated.
    Makefile:99: recipe for target 'bigfile/tests/test_virtmem.t' failed
    make: *** [bigfile/tests/test_virtmem.t] Error 1

In C99 declaration after statement is ok, and we explicitly compile with -std=gnu99.

Python >= 3.4 however adds -Werror=declaration-after-statement even for extension
modules irregardless of their compilation flags:

  https://bugs.python.org/issue21121

and the build fails this way:

    building 'wendelin.bigfile._bigfile' extension
    creating build/temp.linux-x86_64-3.4
    creating build/temp.linux-x86_64-3.4/bigfile
    creating build/temp.linux-x86_64-3.4/lib
    gcc -pthread -Wno-unused-result -Werror=declaration-after-statement -DNDEBUG -fmessage-length=0 -grecord-gcc-switches -O2 -Wall
-D_FORTIFY_SOURCE=2 -fstack-protector -funwind-tables -fasynchronous-unwind-tables -g -DOPENSSL_LOAD_CONF -fPIC -D_GNU_SOURCE
-I./include -I./3rdparty/ccan -I./3rdparty/include -I/usr/include/python3.4m -c bigfile/_bigfile.c -o build/temp.linux-x86_64-3.4/bigfile/_bigfile.o
-std=gnu99 -fplan9-extensions -fvisibility=hidden
    bigfile/_bigfile.c: In function ‘pyfile_new’:
    bigfile/_bigfile.c:679:5: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]
         static char *kw_list[] = {"blksize", NULL};
         ^
    cc1: some warnings being treated as errors
    error: command 'gcc' failed with exit status 1

Ensure we turn off this warning and error because we already rely on compiling in C99 mode.
Reported-and-tested-by: Ivan Tyagov <ivan@tyagov.com>

For this, a small wrapper over py.test is developed (to discover/collect
functions to benchmar, etc) and then it runs such functions several
times in a boxed enveronment.

Benchmarks should be named bench_*.py

We'll use py.test for unit-testing of Python part.

Does similar things to what kernel does - users can mmap file parts into
address space and access them read/write. The manager will be getting
invoked by hardware/OS kernel for cases when there is no page loaded for
read, or when a previousle read-only page is being written to.

Additionally to features provided in kernel, it support to be used to
store back changes in transactional way (see fileh_dirty_writeout()) and
potentially use huge pages for mappings (though this is currently TODO)

We hook into SIGSEGV and handle read/write pagefaults this way.

In this patch there goes stub code that only detects faults and
determines (in arch specific way) whether fault was for read or write
and there is a TODO to pass that information to higher level.

It also comes with tests to detect we still crash if we access something
incorrectly, so people could have coredumps and investigate them.

We'll be testing C code in a similiar-to-python way - keep test_*.c
files nearby and compile/executing them as needed.

Tests are run several times in several ways - as plainly compiled and
also with additional error checkers:

 - AddressSanitizer
 - ThreadSanitizer
 - and similiar checkers from Valgrind

It is an early project decision to use gnu99 & Plan9 C extensions, to
simplify C code.

So far we only build stub wendelin/bigfile/_bigfile.so .

Makefile is introduced because there will be targets which are easier to
handle at make level. For end users no make knowledge is required -
usual `python setup.py build|install|...` work, redirecting to make
where necessary.

As was promised (e870781d "Top-level in-tree import redirector")
setup.py contains install-time hooks to handle in-tree wendelin.py and
install it as a module namespace.

For sdist, we just use `git ls-files` info if we are in a checkout.