mutter-launch: augment with VT and TTY handling

Set the TTY mode appropriately at startup, and clean it up
when the compositor exits. Also, take control of VT switching,
including the calls to drmSetMaster and drmDropMaster as appropriate.
In the future, we the kernel implements the mute evdev ioctl,
we'll also make sure that input devices are appropriately released.

https://bugzilla.gnome.org/show_bug.cgi?id=705861