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

Sun Jan  4 19:24:43 GMT 2009  Matthias Kilian <kili@outback.escape.de>
    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.