rmlock(9) two additions and one change/fix:

• add rm_try_rlock().
• add RM_SLEEPABLE to use sx(9) as the back-end lock in order to sleep while holding the write lock.
• change rm_noreadtoken to a cpu bitmask to indicate which CPUs need to go through the lock/unlock in order to synchronize. As a side effect, this also avoids IPI to CPUs without any readers during rm_wlock.

Discussed with: ups@, rwatson@ on arch@
Sponsored by: Isilon Systems, Inc.