Stop greeter when not needed

This patchset makes gdm stop the greeter session on seat0 when using a Wayland based gdm session together with ENABLE_USER_DISPLAY_SERVER / GDM_SESSION_DISPLAY_MODE_NEW_VT and restarts it again when necessary.

This is versino 2 of this patchset, new this version:

  1. Rework the fix for not starting the greeter when 2 users are logged in (using fast user switching) and one of them logs out.
  2. Only stop the greeter-session if it is using Wayland.

This merge request resolves issue #222 (closed)

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