Don't allow invalid netmail-forwarding addresses

Don't prompt a new user to forward email to their netmail address if they
provided an invalid netmail address (not supported by the system).

If a sysop has an invalid netmail address setup for forwarding, don't try to
forward email (or new user feedback) to that address.

Re-ordered the new user terminal questions a little bit (ask for the backspace
key first, to get earlier manual PETSCII detection). I'm not sure why I was
enabling AUTOTERM along with PETSCII before. Removed that as PETSCII cannot
be auto-detected.