systemd: base "ListenStream" on build options

Make `mpd.socket` configurable based on build options, so that e.g.
`ListenStream=6600` is not added if `tcp` option is not enabled.

systemd/system/meson.build

@@ -12,8 +12,10 @@ if systemd_system_unit_dir == ''
   systemd_system_unit_dir = join_paths(get_option('prefix'), 'lib', 'systemd', 'system')
 endif
 
-install_data(
-  'mpd.socket',
+configure_file(
+  input: 'mpd.socket.in',
+  output: 'mpd.socket',
+  configuration: systemd_socket_conf,
   install_dir: systemd_system_unit_dir,
 )
 

systemd/system/mpd.socket.in

@@ -1,6 +1,5 @@
 [Socket]
-ListenStream=%t/mpd/socket
-ListenStream=6600
+@listen_streams@
 Backlog=5
 KeepAlive=true
 PassCredentials=true