MPSAFE - Add a set of general blocking/spinnable mutex functions.

MPSAFE - Add a set of general blocking/spinnable mutex functions.

These locks are intended to eventually replace lockmgr locks for most
use cases.

* Optimized based on in-line atomic_cmpset_int() calls with fallback call.

* Recursive shared and exclusive locks.  Downgrading, and non-blocking
  upgrading.

* Interlocked wakeup flags.

* Serial wakeup for exclusive waiters (i.e. optimal in the face of a large
  number of waiting threads).  Mass-wakeup for shared waiters.

* Additional entry points for spinning.

* Ref-count support, separate from lock count.