include/{mutex,wmutex}: Wait forever if timeout is 0 or missing

include/{mutex,wmutex}: Wait forever if timeout is 0 or missing

The mutex_lock() and wmutex_lock() functions return immediately if
the timeout is set to 0 or is missing. However, to be compatible with
the previous timeout-less implementations, the functions must wait
forever if no timeout was specified.
This commit changes the two functions so they wait forever if the
timeout is 0 or missing.