Bring back tty-mode in an xterm login option (a la F2)

[jdreed]

Athena 9 had the option to use a "terminal style login session", which basically ran xterm -ls for you. We should restore that.

[jdreed]

I've committed a change to debathena-xsession which does the following:

• Creates a new session called "Athena TTY Mode" (the name is negotiable)
• Add /etc/X11/Xsession.d/00debathena-gdm-sucks, which checks for that session type, kills pulseaudio if you're over quota, runs xterm -ls (so you get the Athena tty login sequence), and then exits.

Note that if we change the session name, we need to change it in the message text in 01debathena-quota (see #600)