Adding systemd unit files for socket activation

- Support for multiples concurrent instances, templated on
  WAYLAND_DISPLAY
- Use standard input for the socket file descriptor (inetd style)
- Always use the socket provided by systemd with the systemd user
  service
- wayland-instance@.target is intended to be a special target a bit like
  graphical-session.target.

foot-server@.service.in

@@ -0,0 +1,13 @@
+[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)
+
+[Install]
+WantedBy=wayland-instance@.target