Introduce a new kind of mutex, the suspension safe one.

Introduce a new kind of mutex, the suspension safe one.

	* mono-mutex.c: Add mono_mutex_init_suspend_safe so special targets
	like OSX that have broken implementations of suspend for pthread mutex
	will hang less.

	A suspension safe mutex means one that can handle this scenario:

	mutex M

	thread 1:
	1)lock M
	2)suspend thread 2
	3)unlock M
	4)lock M

	thread 2:
	5)lock M

	Say (1) happens before (5) and (5) happens before (2).
	This means that thread 2 was suspended by the kernel because
	it's waiting on mutext M.

	Thread 1 then proceed to suspend thread 2 and unlock/lock the
	mutex.

	If the kernel implements mutexes with FIFO wait lists, this means
	that thread 1 will be blocked waiting for thread 2 acquire the lock.
	Since thread 2 is suspended, we have a deadlock.

	A suspend safe mutex is an unfair lock but will schedule any runable
	thread that is waiting for a the lock.

	This problem was witnessed on OSX in mono/tests/thread-exit.cs.