units: let's set a socket name for networkd rtnl socket

Let's make our networkd sockets recognizable purely by name. It hink
already for debugging it's a good idea to always set socket names, in
particular for services that have multiple sockets they listen on.

This adds a name to the rtnl socket, which so far missed one. Note that
the C code won't look for it, for compat with older versions, but at
least things are a bit more debuggable.

units/systemd-networkd.socket

@@ -18,6 +18,7 @@ Conflicts=shutdown.target
 [Socket]
 ReceiveBuffer=128M
 ListenNetlink=route 1361
+FileDescriptorName=rtnl
 PassPacketInfo=yes
 
 [Install]