GDM: Put itself on screen when homed locks

Split out from #3 (closed)

See the research document and this other section for details, but TL;DR:

• GDM should detect when the user is looking at a VT of a secure-locked session and forcibly activate the greeter session. Reason: a secure-locked session is frozen so if the user accidentally ends up there they'll be stuck until reboot
• Make systemd-homed send a signal whenever a user is locked, which GDM can respond to by putting itself on screen
• GDM should put itself on screen by just switching to greeter session, then pre-selecting a user