Ties systemd units to graphical-session.target

- wayland-instance template target was a mistake.
  Systemd does not support simultaneous same user session, so stop
  trying to go against that.
- Only start systemd units in Wayland environments.

meson.build

@@ -283,13 +283,13 @@ if systemd.found() or custom_systemd_units_dir != ''
 
   configure_file(
     configuration: configuration,
-    input: 'foot-server@.service.in',
+    input: 'foot-server.service.in',
     output: '@BASENAME@',
     install_dir: systemd_units_dir
     )
 
   install_data(
-    'foot-server@.socket',
+    'foot-server.socket',
     install_dir: systemd_units_dir)
 endif