get rid of version checks

version checks don't work at all on kernels with backported bpf bits
they also fail when /usr/include/linux/bpf.h doesn't match loaded
kernel.
Fix these issues by embedding bpf.h into libbcc.so and force load it
in clang, so we can remove all version checks and rely on verifier
complaining on unknown function call.
Later patch can make verifier errors less cryptic by converting
'unknown call 12' to strings.

while at it update bpf.h to the latest.
Signed-off-by: Alexei Starovoitov <ast@fb.com>

Bug fix around USDT argument handling for constants

For USDT arguments of the form "-4@$-1", the parsing logic would fail
because it didn't expect the second number (the actual value of the
USDT argument as a constant) to be negative. This is now fixed.

IPv6 strings with an inet6_ntoa

Replaced `which --skip-alias` with `ProcUtils.which`

Support for global variable arguments in USDT probes

Lua Tools for BCC

Percpu support

I previously thought it wouldn't be possible to probe arbitrary user-mode addresses,
but it turns out that it is. This was required to support global variable arguments
in USDT probes. For example, there are a few probes in libpthread whose arguments
refer to the mp_ global variable. These are now supported.

Additionally, refactored process-related utils into a ProcUtils class in procstat.py,
and implemented a version of `which` (`ProcUtils.which`) that is as portable as
possible -- it only relies on traversing the PATH environment variable.