Revert "Use fmutex_open for 100% locking and auto-removal upon close/termination"

This reverts commit e55183c0.

fmutex_open()'s auto-remove feature doesn't work on normal/local Linux file
systems: unlink() of a file immediately removes its directory entry allowing
subsequent opens of the same file name, defeating the lock.

I had tested fmutex_open() on a Samba mount (from Linux) and it worked as I
wanted (like the Windows version does). Still searching for a good solution:
if a process terminates, gracefully or not, I'd like the file to be removed
at that time and until then, other processes or threads can't create or open
the same lock file (using O_EXCL and file locking to insure this).