Reserve local port in the pool in multicast

Submitted by Xavier Claessens @xclaesse

Link to original bug (#770969)

Description

In the multicast case, we bind the socket on ANY address, so I guess the port should be taken from the unicast pool as well as from the multicast pool.

I think alloc_ports_one_family() should reserve a multicast address from the pool, then try to reserve the same port in the unicast pool, then loop until it finds a port that is available in both pools.

I added FIXME comments in gst_rtsp_stream_get_multicast_address() and gst_rtsp_stream_reserve_address() about this.