Add Listen* to dbus properties

sockets.socket - Test
	  Loaded: loaded (/home/alxchk/.config/systemd/user/sockets.socket; static)
	  Active: inactive (dead)
	  Listen: Stream: /tmp/stream1
	          Stream: @stream4
	          Stream: [::]:9999
	          Stream: 127.0.0.2:9996
	          Stream: [::1]:9996
	          Datagram: /tmp/stream2
	          Datagram: @stream5
	          Datagram: [::]:9998
	          Datagram: 127.0.0.2:9995
	          Datagram: [::1]:9995
	          SequentialPacket: @stream6
	          SequentialPacket: /tmp/stream3
	          FIFO: /tmp/fifo1
	          Special: /dev/input/event9
	          Netlink: kobject-uevent 0
	          MessageQueue: /msgqueue1

[zj: - minor cleanups,
     - free i.listen,
     - remove sorting, because the order or sockets matters.]