Several improvements to rmlock(9). Many of these are based on patches
provided by Isilon.

• Add an rm_assert() supporting various lock assertions similar to other locking primitives. Because rmlocks track readers the assertions are always fully accurate unlike rw_assert() and sx_assert().
• Flesh out the lock class methods for rmlocks to support sleeping via condvars and rm_sleep() (but only while holding write locks), rmlock details in 'show lock' in DDB, and the lc_owner method used by dtrace.
• Add an internal destroyed cookie so that API functions can assert that an rmlock is not destroyed.
• Make use of rm_assert() to add various assertions to the API (e.g. to assert locks are held when an unlock routine is called).
• Give RM_SLEEPABLE locks their own lock class and always use the rmlock's own lock_object with WITNESS.
• Use THREAD_NO_SLEEPING() / THREAD_SLEEPING_OK() to disallow sleeping while holding a read lock on an rmlock.

Submitted by: andre
Obtained from: EMC/Isilon