Factor out ssh_hostport_setup().

Factor out ssh_hostport_setup().

This is the part of ssh.c's connect_to_host() which figures out the
host name and port number that logically identify the connection -
i.e. not necessarily where we physically connected to, but what we'll
use to look up the saved session cache, put in the window title bar,
and give to the connection sharing code to identify other connections
to share with.

I'm about to want to use it for another purpose, so it needs to be
moved out into a separate function.