Move SSH authentication into answer()

This should fix a long-standing issue where someone could connect to
the SSH port and do nothing, which would prevent other incoming
terminal sessions from being accepted until it times out.

Unfortunately, this means that Synchronet can't send any data until
authentication is completed, which means useful messages about why
you're being disconnected (ie: "Sorry, all terminal nodes are in use or otherwise unavailable.")
as well as usless information nobody ever cares about (ie: The IP
you're connecting from, that it is resolving your hostname, etc).
can no longer be sent to the user.