frames: Quit immediately on SIGTERM

Restarting a X11 window manager is a busy process, trying to leniently quit the main loop may result in old and new instances each having a frames client up and running, and the window handover to be less clean than it should due to the frames client that is about to exit still being able to react to the batch of events resulting from the window manager switch that is already undergoing.

In order to avoid extending this transition period any long, make the frames client exit() the process immediately when SIGTERM is gotten from the parent process.