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.
Yes, the handlers re-register themselves because some obscure antique Unix (I suspect SunOS ... ehm, I mean Slowlaris ... but I can't remember for sure) clears the signal handler when invoking it, i.e. (as you guessed) resets it to SIG_DFL.

As to why signal() and not sigaction() ... yes, that's one of the things I hope to change some day, but first we have to review whether every currently-supported platform actually has it.  I think they do, but we need to check; and then we need to actually make the changes, which isn't urgent given that signal() kinda works.  Obviously, a working and tested patch would help that process along ;^>