freeze googletest version

We have googletest as a dependency for our tests. This dependency is
defined by cmake and fetched when we run make, but we were always
fetching master, which means if there is a breaking change upstream our
build will break without notice (which happened recently). To avoid
this, we freeze the googletest version to the last stable, and once
there's a new googletest release we can upgrade it here (making changes
to our code if necessary).

Fixes: https://github.com/iovisor/bpftrace/issues/122

add make install target

enforce infinite rlimit to avoid problems when allocating maps

update tutorial with tracepoint args

Destination can be set through the CMAKE_INSTALL_PREFIX variable when
running cmake (for example, `-DCMAKE_INSTALL_PREFIX=/usr/local`).

Files are installed as described below:
  - `build/src/bpftrace` -> `${CMAKE_INSTALL_PREFIX}/sbin`
  - `tools/*` -> `${CMAKE_INSTALL_PREFIX}/share/bpftrace/tools`
  - `man/*` -> `${CMAKE_INSTALL_PREFIX}/man`

Fixes: https://github.com/iovisor/bpftrace/issues/16

allow u32 binops and map keys

switch some tools to use args

Tracepoint format parser

Adds a new builtin "args", which acts as a pointer to the tracepoint
arguments struct when inside tracepoint probes.

fix string comparison

This patch fixes strcmp by reimplementing it as a IRBuilder method and
unrolling the loop based on the literal string size instead of
STRING_SIZE.

Fixes: https://github.com/iovisor/bpftrace/issues/14

truncate zero range on hist

fix BEGIN / END probes

add .gitignore

add verbose debug output with -dd

Docs

update usdt with arguments examples

BEGIN/END probes were broken after
https://github.com/iovisor/bpftrace/pull/100.

Fixes: https://github.com/iovisor/bpftrace/issues/51

Ignore common build directories. Useful to navigate the code base with
tools taking .gitignore into account.

LLVM optimizes IR before turning it into BPF instructions. Sometimes the
optimization might cut off important code if the IR was not built
correctly. -d is not enough to find these issues, therefore this patch
introduces more debug levels to also print the raw IR before it gets
optimized.

 usdt: resolve usdt arguments

Now we can infer the path from current_attach_point_, which means we
don't need to keep track of both references.

This patch introduces initial support for USDT arguments through the
argX builtins.  For example:
`./bpftrace -e 'usdt:bin:probe { printf("%s\n", str(arg0)); }'`

Ref: https://github.com/iovisor/bpftrace/issues/33

fix attach to tracepoint regression

Regression introduced by https://github.com/iovisor/bpftrace/pull/100.

We can attach to multiple tracepoints within the same process without
probem, which means we don't need to a suffix to it.

Fixes: https://github.com/iovisor/bpftrace/issues/103

add system() call