Introduce some WITNESS improvements:

- Speedup the lock orderings lookup modifying the witness graph from a
  linked tree to a matrix. A table lookup caches the lock orderings in
  order to make a O(1) access for them. Any witness object has an unique
  index withing this lookup cache table.
- Reduce the lock contention on w_mtx acquiring it only when the LOR
  actually happens and not in a sane case. In order to do this don't totally
  flush lock lists (per-CPU spinlocks list and per-thread sleeplocks list)
  but check for ll_count anytime we need to have to verify allocations sanity.
- Introduce the function witness_thread_exit() in the witness namespace which
  should verify a thread doesn't hold any witness occurrence why exiting.
- Rename the sysctl debug.witness.graphs into debug.witness.fullgraph and
  add debug.witness.badstacks which prints out stacks for LOR revealed.
  This is implemented using the stack(9) support, which makes WITNESS to be
  dependent by the STACK option or by the DDB (including STACK) option.
- Fix style(9) for src/sys/kern/subr_witness.c

The hash table approach has been developed by Ilya Maykov on the behalf of
Isilon Systems which kindly released the patch.
Jeff Roberson, ported the patch to -CURRENT and fixed w_mtx contention, on the
behalf of Nokia.

Submitted by:	Ilya Maykov <ivmaykov at gmail dot com> (Isilon Systems), jeff
Sponsored by:	Nokia

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