Refine UMA bucket allocation to reduce space consumption and improve
performance.

 - Always free to the alloc bucket if there is space.  This gives LIFO
   allocation order to improve hot-cache performance.  This also allows
   for zones with a single bucket per-cpu rather than a pair if the entire
   working set fits in one bucket.
 - Enable per-cpu caches of buckets.  To prevent recursive bucket
   allocation one bucket zone still has per-cpu caches disabled.
 - Pick the initial bucket size based on a table driven maximum size
   per-bucket rather than the number of items per-page.  This gives
   more sane initial sizes.
 - Only grow the bucket size when we face contention on the zone lock, this
   causes bucket sizes to grow more slowly.
 - Adjust the number of items per-bucket to account for the header space.
   This packs the buckets more efficiently per-page while making them
   not quite powers of two.
 - Eliminate the per-zone free bucket list.  Always return buckets back
   to the bucket zone.  This ensures that as zones grow into larger
   bucket sizes they eventually discard the smaller sizes.  It persists
   fewer buckets in the system.  The locking is slightly trickier.
 - Only switch buckets in zalloc, not zfree, this eliminates pathological
   cases where we ping-pong between two buckets.
 - Ensure that the thread that fills a new bucket gets to allocate from
   it to give a better upper bound on allocation time.

Sponsored by:	EMC / Isilon Storage Division
Merge the 3-way merge marker tweak.
Merge the commit template patch.