- Add an api for doing smp safe locks in userland.

 - Add an api for doing smp safe locks in userland.
 - umtx_lock() is defined as an inline in umtx.h.  It tries to do an
   uncontested acquire of a lock which falls back to the _umtx_lock()
   system-call if that fails.
 - umtx_unlock() is also an inline which falls back to _umtx_unlock() if the
   uncontested unlock fails.
 - Locks are keyed off of the thr_id_t of the currently running thread which
   is currently just the pointer to the 'struct thread' in kernel.
 - _umtx_lock() uses the proc pointer to synchronize access to blocked thread
   queues which are stored in the first blocked thread.