This moves the main loop out of the macosx interface module.
Instead, the open callback only initializes the interface and the
close callback cleans up stuff. The mainloop is now started in
darwinvlc.m. In case the mac interface is not used, a CoreFoundation
mainloop is solely started to wait for termination events.

Additionally, this cleans up darwinvlc.m, and fixes signal
handling, which was dead code so far (in case the interface was used).
Now, GCD is used to catch SIGINT and SIGTERM in order to allow
ordinary shutdown.

refs #14362
close #6354