Always check the result of pthread_mutex_lock() and pthread_mutex_unlock().

Always check the result of pthread_mutex_lock() and pthread_mutex_unlock().

Don't check pthread_mutex_*lock() only on Linux and/or only if DEBUG
is defined. The return values of those functions are well defined
and should be supported on all operation systems with pthreads. The
checks are cheap enough to do them even in the default build (without
-DDEBUG).

While here, recycle an unused macro ASSERT_LOCK_NOTHELD, and let
the debugBelch part enabled with -DLOCK_DEBUG work independently
of -DDEBUG.