Set non-blocking flag on all FDs

Things seems to work just fine even without this, but let's stay on
the safe side and set the flag.
Quit if user presses Ctrl-C

It is now possible to terminate the server by sending SIGINT to crash.
The first Ctrl-C will try to shut down clients gracefully. If that
doesn't help, hit Ctrl-C a second time and it will close connections
forcibly.