mutter-launch: use systemd to obtain the TTY

Using the command line or an environment variable is dangerous,
as those can be spoofed to gain access to other sessions.

https://bugzilla.gnome.org/show_bug.cgi?id=705861
[maybe?] Bump version to 3.9.90

Keep the version in sync with the master branch.
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,
and make sure that the compositor calls drmDropMaster when switched
away.
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
wayland: use the new mutter-launch capabilities to handle TTY and VT

Integrate MetaTTY with meta-weston-launch, into a new MetaLauncher
helper, that takes care of talking with the setuid helper, calling
set/dropMaster as appropriate and opening the evdev devices when
asked by clutter.
In the future, this object could be made to talk to logind instead
of our private helper, or the helper could be turned into a systemd
daemon.

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