Fix GC allocation mutex in child after a fork

Fix GC allocation mutex in child after a fork

Even though after a fork the child only inherits the single thread
that called the fork(), if another thread in the parent was attempting
to lock the mutex while being held in fork_child_prepare(), the mutex
will be left in an inconsistent state in the child after the UNLOCK().
This is the case, at least, in Mac OS X and leads to an unusable GC in
the child which will block when attempting to perform any GC operation
that acquires the mutex.

* pthread_support.c [CAN_HANDLE_FORK && USE_PTHREAD_LOCKS]
(fork_child_proc): Add assertion (after UNLOCK) that the lock is not
held actually, and, then, re-initialize GC_allocate_ml; add comments.