add support for serverside safe shutdown

[novedevo]

we can't close any connections without sending 421 ( we can send asynchronously if necessary), so we should provide a way to shutdown the program (perhaps responding to kill -9 signals or something) that sends 421s to all the clients and saves any necessary information to disk before closing

[novedevo]

RFC-compliant (see d789975 and related commits), but more processes should probably check for signals (e.g. test for signals while waiting for a DNS resolution)

[novedevo]

b25aa3e is relevant