Merge branch 'welash/frotz-mac_sema'

Merge branch 'welash/frotz-mac_sema'

Because MacOS lacks support for unnamed POSIX semaphores, this wrapper
code was added.  For MacOS, it uses Grand Central Dispatch semaphores,
otherwise standard POSIX unnamed semaphores are used.  Currently return
codes are not checked.  Eventually this code will be gone over and
turned into a generic drop-in for other projects needing to reconcile
unnamed semaphores with the Apple Way.  At that point, structure and
function names will be changed and return codes will be checked.
Because this is intended for later widespread re-use, ux_sema.h is
dual-licensed GPL and MIT.