Forced commit, for previous revision.

Make state transitions of a thread on a mutex queue
atomic (with respect to other threads and signal handlers).
This includes:
	o Introduce two functions to implement atomicity with respect
	  to other threads and signal handlers. Basically,
	  _thread_critical_enter() locks the calling thread and blocks
	  signals. _thread_critical_exit() unblocks signals and unlocks
	  the thread.

	o Introduce two new functions:
	  get_muncontested() locks a mutex that is not owned by
	  another thread.
	  get_mcontested() places a thread on a contested mutex's
	  queue, taking care to use the _thread_critical_enter/exit
	  functions to protect thread state.

	o Modify mutex_unlock_common() to also protect state transitions.
	  In this case it needs the cooperation of mutex_queue_deq(), which
	  must return with the thread locked and signals disabled *before*
	  it takes the thread off the queue.

Combine _pthread_mutex_lock() and _pthread_mutex_trylock()
into one function: mutex_lock_common(), that can handle
both cases. Its behaviour is controlled by an argument,
int nonblock, which if not zero means do not attempt
to acquire a contested mutex if the uncontested case fails.

BTW, when I write about contested and uncontested mutexes, I'm writing
about it from the application's point of view. I'm not writing about
internal locking of pthread_mutex->lock, which is achieved differently.

While internal mutex locking is mostly done, there's still a bit more
work left in this area.

Approved by:	markm/mentor, re/blanket libthr
Reviewed by:	jeff (slightly diff. revision)

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