Update systemd service files

- add ConditionEnvironment so that it only runs if WAYLAND_DISPLAY
  environmental variable exists.
- replace wayland-instance@.target to the generic
  graphical-session.target. Not sure about this btw but I feel like
  wayland-instance@.target is not needed.
- remove `Requires=%N.socket` in [Unit]. Documentations says "No implicit WantedBy= or RequiredBy= dependency from the socket to the service is added. This means that the service may be started without the socket, in which case it must be able to open sockets by itself. To prevent this, an explicit Requires= dependency may be added."
- add `Also=%N.socket` in [Install].
- replace wayland-instance@.target to generic sockets.target in
  foot-server@.socket.

foot-server@.service.in

@@ -1,13 +1,17 @@
-[Service]
-ExecStart=@bindir@/foot --server=0
-Environment=WAYLAND_DISPLAY=%i
-NonBlocking=true
-StandardInput=socket
-
 [Unit]
-Requires=%N.socket
 Description=Foot terminal server mode for WAYLAND_DISPLAY=%i
 Documentation=man:foot(1)
+PartOf=graphical-session.target
+After=graphical-session.target
+ConditionEnvironment=WAYLAND_DISPLAY
+
+[Service]
+ExecStart=@bindir@/foot --server=0
+Environment="WAYLAND_DISPLAY=%i"
+NonBlocking=true
+StandardInput=socket
+Restart=on-failure
 
 [Install]
-WantedBy=wayland-instance@.target
+Also=%N.socket
+WantedBy=graphical-session.target

foot-server@.socket

@@ -1,5 +1,8 @@
+[Unit]
+Description=Foot server socket
+
 [Socket]
 ListenStream=%t/foot-%i.sock
 
 [Install]
-WantedBy=wayland-instance@.target
+WantedBy=sockets.target