wayland: add TTY and DRM master management

Now that we have a setuid launcher binary, we can make use of
using a private protocol through the socket we're passed at startup.

We also use the new hook in clutter-evdev to ask mutter-launch for
the FDs of the input devices we need, and we emulate the old X
DRM lock with a nested GMainContext without sources.

In the future, mutter-launch will be replaced with the new logind
API currently in development.

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