Add umtx.c, a simple utility which implements userland mutexes using

Add umtx.c, a simple utility which implements userland mutexes using
the new umtx_*() system calls.  Compile normally and play around, e.g.:

umtx -v -t 10 /tmp/lock.dat sleep 5
umtx -v -t 10 /tmp/lock.dat ls

The test code includes some simple ^C cleanup code as well, though I
don't block signals during the global variable fixup so there is a race in
the ^C cleanup code.