networkd: support socket activation

Still keep the non-socket activation code around for starting from the commandline, but
will likely drop that too in the future.

units/systemd-networkd.socket

@@ -0,0 +1,20 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=networkd rtnetlink socket
+Documentation=man:systemd-networkd.service(8) man:rtnetlink(7)
+DefaultDependencies=no
+Before=sockets.target
+
+[Socket]
+ReceiveBuffer=8M
+ListenNetlink=route 273
+PassCredentials=yes
+
+[Install]
+WantedBy=sockets.target