include/{mutex,wmutex}: Change mutexes back to the sleep-based approach

include/{mutex,wmutex}: Change mutexes back to the sleep-based approach

Using inotifywait to wait on a mutex does not work reliably when the mutex
is on an NFS and the processes synchronizing on it are running on different
machines.
This commit changes the implementations of mutex and wmutex back to use
sleep instead of inotifywait.