Merge pull request #2816 from xmcclure/profile-clean-0

Merge pull request #2816 from xmcclure/profile-clean-0

Various cleanups on profiler

3 commits:

Assist on profiler module loading

- Add a MONO_PROFILER_LIB_DIR from which profiler modules are
  dynamically loading, in case it is not the normal library path
- Add information about profiler loading to mono_trace

Comments and clarity in proflog.c

- Add comments on structures
- Rename 'data' and 'data_end' to 'cursor' and 'buf_end' in LogBuffer/StatsBuffer as the old names were confusing
- Add new assert when processing samples

Improve a safety check when writing data into StatBuffer

- The safety check should occur such that if the new value for
StatBuffer::cursor is beyond the bounds of the StatBuffer, the cursor
is not updated.