Improve UMTX_PROFILING:
- Use u_int values for length and max_length values
- Add a way to reset the max_length heuristic in order to have the
  possibility to reuse the mechanism consecutively without rebooting
  the machine
- Add a way to quick display top5 contented buckets in the system for
  the max_length value.
  This should give a quick overview on the quality of the hash table
  distribution.

Sponsored by:	EMC / Isilon storage division
Reviewed by:	jeff, davide
Fix another compiler warning issue when invariants are disabled.