Special handling of signals for cf-key.

Handle SIGINT and SIGTERM to set gobal flag that makes the
ShowHost callback return false, thus causing ScanLastSeenQuality break
out of the while loop and return cleanly.
Thank you for the explanation and clarification.

sigaction() is defined in POSIX and I'd hope it would be as universally available as, say, something like chmod() on all unixes and unix-like :-) Alas you are correct though: we live in a world where most systems are not 100% POSIX compliant so this probably needs to be vetted case by case.

Windows has no sigaction() equivalent so I'm _guessing_ that's why signal() was originally used. I did find [this implementation](http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=lib/sigaction.c;h=c56ca46028e33a41e4da1780e09632fcaa0fda58;hb=HEAD) "tailored to native Windows behavior" by the `gnulib` folks however and it may be helpful for a home-grown effort in libcompat/ it the switch is ever attempted... or perhaps even included as-is. Posted here merely for future reference and those curious.