Fix: various fixes

Fix tests:

- Coding style,
- Error checking,
- Missing return values in some main exit paths,
- Remove memory allocation in lttng-ust-benchmarks.c. We just need to
  pass arguments starting from offset 1 to do a "barrel shifting" of the
  arguments.
- 10E10 -> 1E10 math error,
- Use CLOCK_MONOTONIC instead of gettimeofday(), since it is not
  sensitive to major NTP updates. It returns ns rather than us.
- This is a benchmark, so O2 compilation flag should be used rather than
  O0,
- This is a benchmark, so define NDEBUG (disable asserts),
- Need to override a Makefile implicit rule to break a circular
  dependency,
- Add side-effect to basic benchmark test, needed for
  !defined(WITH_UST).
- Don't depend on babeltrace: we can do this kind of benchmark more
  precisely by using the "snapshot" flight recorder tracing, and knowing
  the number of events we push into the buffer. There is therefore no
  consumer, and no events are discarded. This depends on updating
  lttng-tools to master, which includes "create_snapshot" in the python
  bindings.
- sha2 benchmark: since we want to know how many times the tracepoint is
  hit (to do the snapshot tracing without babeltrace), change this test
  so we call sha2 in a loop, along with the tracepoint. Call this loop
  1M times. Remove tracepoints from within sha2.c. All we care about in
  this test is that there is some non-trivial activity done by the
  application between tracepoints,
- Update gitignore.