"How do you uniformly and asynchronously request that a process clean up its resources and exit, when e.g. the hardware is about to shut off?" That doesn't sound like a bad question to me.

the only time i've made good use of signals is for exiting. you can catch ctrl-c that way, so your app isn't forced to exit without being able to clean up. and you can install a SIGQUIT handler to take action when the os wants you to shut down.