Resolve bind() endpoint in udpserver, allowing to bind to hostname or ipv6 addresses

Main goal is easy AF_INET6 compatibility, so that e.g. `--collectd-ip=ipv6.mydomain.tld` or `--collectd-ip=::` will do the right thing.

Shouldn't break anything wrt ipv4, I think, except if socket.bind() resolved names to ipv4 before (not sure if python does that) *and* name was used with udpserver --something-ip option *and* gai.conf (or whatever libc resolver) is (mis-)configured to yield ipv6 first *and* that name has AAAA record.
Should definitely work as before with default 127.0.0.1 binds.