Added --listen option to set address to bind to

Wanted to be able to run the server on only the irc.domain interface; it was an easy hack.  It seemed obvious that when an explicit interface was given it should resolve that name, rather than the host's base (it has half a dozen role-based addresses).

There are other names possible for the option, but listen won my internal straw poll as the most common.  There are various extensions I had no urge to implement, such as a list of addresses, or allowing addr:port here, etc.