mctp-demux-daemon: Use systemd socket activation

Take advantage of lazy initialisation.

However, this also allows (one) daemon providing services over MCTP
(pldmd) an opportunity to start prior to MCTP interfaces coming up. This
is a stunted way to provide capabilities that might be assumed by other
components (the host) without implementing some required messages from
the MCTP standard.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I4b1c21f4fd42f84e2c85a453570a74330fc70ecf
diff --git a/systemd/system/mctp-demux.socket b/systemd/system/mctp-demux.socket
new file mode 100644
index 0000000..8b42b5d
--- /dev/null
+++ b/systemd/system/mctp-demux.socket
@@ -0,0 +1,9 @@
+[Unit]
+Description=MCTP demux Unix domain socket
+
+[Socket]
+ListenSequentialPacket=@mctp-mux
+Accept=no
+
+[Install]
+WantedBy=sockets.target