Make sure that the target user is able to join a local channel

Implement ERR_USERNOTONSERV(504) numeric and make sure that the
target user is on the same server when inviting other users to
local ("&") channels.

ircd-ratbox uses the ERR_USERNOTONSERV(504) numeric for this, and I
think this is a good idea -- other IRC daemons (like ircu) silently
drop such impossible invites, but thats not a big benefit ...

Idea by Cahata, thanks! Closes #183.