- Add a BIT_FFS() macro and use it to replace cpusetffs_obj()

Discussed with:	attilio
Sponsored by:	EMC / Isilon Storage Division
 - Convert the slab free item list from a linked array of indices to a
   bitmap using sys/bitset.  This is much simpler, has lower space
   overhead and is cheaper in most cases.
 - Use a second bitmap for invariants asserts and improve the quality of
   the asserts as well as the number of erroneous conditions that we will
   catch.
 - Drastically simplify sizing code.  Special case refcnt zones since they
   will be going away.
 - Update stale comments.

Sponsored by:	EMC / Isilon Storage Division