Fix a bug with signaling a thread that waits for condvar

Fix a bug with signaling a thread that waits for condvar

* src/thread.c (lisp_mutex_lock_for_thread): New function,
with all the guts of lisp_mutex_lock.
(lisp_mutex_lock): Call lisp_mutex_lock_for_thread.
(condition_wait_callback): Don't call post_acquire_global_lock
before locking the mutex, as that could cause a signaled thread to
exit prematurely, because the condvar's mutex is recorded to be
not owned by any thread, and with-mutex wants to unlock it as part
of unwinding the stack in response to the signal.