back-end DRM: Setting up seat / create a fake for systems without input device

This MR was initially about having a way to map surfaces in kiosk-shell, even if there's no seat available in the system, because we've changed the way we handle activation.

After some discussions, we noticed that maybe it would be a better idea to just use a fake seat rather having to deal with other potential issue this changes was doing.

Further more, I've discovered an issue with specified a seat over the command line and added to this series.

Specifically, this MR now does two things:

• takes a --seat into consideration when passed over cmd line. This also allows setting up a fake seat.
• implicitly, create a fake seat if no the user doesn't specify a seat (the second patch).

This was prompted by #587 (closed).

There isn't a lot going on at this point, we use kiosk_shell and hang out the focused_surface and avoid any keyboard input focus if kiosk_shell is not set.

/cc @pq