BPFTRACE_KERNEL_SOURCE  environment variable

* add bpftrace man page

* changes to bpftrace man page

* fix bpftrace manpage file name

Still continue regular clang parsing, just without extra kernel header flags

The previous fix was bad as it elided a correct early return.

Supress invalid file warning for wildcarded tracepoints

Fix name of kernel required config option in INSTALL.md

Wildcards, unless expanded, are not valid tracepoints. We should not
warn about missing files if the patch is wildcarded. Interestingly enough,
the previous behavior (early return) could have caused fewer tracepoints
to be attached than available. However in practice, this doesn't seem
to happen.

This closes issue #133.

In Linux Kernel 4.7, CONFIG_HAVE_BPF_JIT was splitted into two
(CONFIG_HAVE_EBPF_JIT and CONFIG_HAVE_CBPF_JIT) for distinguishing cBPF
and eBPF JITs
(https://www.mail-archive.com/netdev@vger.kernel.org/msg110538.html)

Fix typos

Add filter by cgroup-v2 id

INSTALL.md: add clang-devel as Fedora build dep

To get the cgroup id, you can use the following program to call
name_to_handle_at(), borrowed from
linux/tools/testing/selftests/bpf/cgroup_helpers.c:
```
wget https://gist.githubusercontent.com/alban/4a9aea9636d9fdf11709eddd21df4cc4/raw/f4fc915483a069d6044957e27df091923f95c48b/getcgroupid.c
gcc -Wall -o getcgroupid getcgroupid.c
./getcgroupid /sys/fs/cgroup/unified/
0x100000001
4294967297
```

To test it, I run the tracer in one terminal:
```
./build.sh
sudo ./build-release/src/bpftrace -e 'tracepoint:syscalls:sys_enter_openat /cgroup == 0x100000001/ { printf("%s\n", str(args->filename)); }'
```

And some tests in another terminal:
```
sudo -s
echo $$ > /sys/fs/cgroup/unified/cgroup.procs
cat /dev/null
```

And observe the output of the tracer:
```
Attaching 1 probe...
/etc/ld.so.cache
/lib64/libc.so.6
/usr/lib/locale/locale-archive
/dev/null
^C
```

List clang-devel as a build dependency on Fedora.
Otherwise 'cmake' fails with:

```
CMake Error at CMakeLists.txt:67 (find_package):
  By not providing "FindClang.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by "Clang", but
  CMake did not find one.

  Could not find a package configuration file provided by "Clang" with any of
  the following names:

    ClangConfig.cmake
    clang-config.cmake

  Add the installation prefix of "Clang" to CMAKE_PREFIX_PATH or set
  "Clang_DIR" to a directory containing one of the above files.  If "Clang"
  provides a separate development package or SDK, be sure it has been
  installed.
```

Remove llvm, llvm-libs since llvm-devel already pulls them in.

fixed some small errors

add fedora installation instructions to readme

update tutorial with kprobe struct args

fixes: https://github.com/iovisor/bpftrace/issues/8

Update README/INSTALL

filename typo

Tools

To get full arguments to pass into Clang for reading kernel headers

cmake: rename gtest external project target