wayland: Reimplement wl-drm

As of nowadays, most clients have switched to the linux-dmabuf protocol,
except Xwayland, which still needs the wl-drm protocol.

On the other hand, we would like to unify some buffer handling code.
There are a few options:

- drop the support for wl-drm protocol. Not doable, because Xwayland
  still needs it, even though it uses the linux dmabuf feedback protocol
  too
- re-implement the wl-drm protocol
- re-implement the minimal part of the wl-drm protocol needed by
  Xwayland

This change goes after the third option. Only the node name and the
capabilities will be sent. The buffer factory requests are not
implemented, but they can be if we discover that some clients need them.