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.
Maybe I'm just totally misinterpreting but, for my own benefit, would someone please be kind enough to explain this to me? ThisAgentInit() registers handleSingal() as a handler for SIGINT and SIGTERM but then it seems said handler re re-registers the itself, presumably because signal() resets to SIG_DFL (???). Also why signal() and not sigaction()?