Remove vfs_mount(9), it have been died since 2004.

Noticed by:	git via alfred
MFC after:	2 weeks
 - Add a new UMA API: uma_zcache_create().  This makes a zone without any
   backing memory that is only a container for per-cpu caches of arbitrary
   pointer items.  These zones have no kegs.
 - Convert the regular keg based allocator to use the new import/release
   functions.
 - Move some stats to be atomics since they would require excessive zone
   locking/unlocking with the new import/release paradigm.  Make
   zone_free_item simpler now that callers can manage more stats.
 - Check for these cache-only zones in the public APIs and debugging
   code by checking zone_first_keg() against NULL.

Sponsored by:	EMC / Isilong Storage Division
- add my mentor -> mentee relation with koobs
- add myself