gst/udp/gstudpsrc.*: Add property to control automatic join/leave of multicast groups.

gst/udp/gstudpsrc.*: Add property to control automatic join/leave of multicast groups.

Original commit message from CVS:
* gst/udp/gstudpsrc.c: (gst_udpsrc_class_init), (gst_udpsrc_init),
(gst_udpsrc_create), (gst_udpsrc_set_property),
(gst_udpsrc_get_property), (gst_udpsrc_start), (gst_udpsrc_stop):
* gst/udp/gstudpsrc.h:
Add property to control automatic join/leave of multicast groups.
Add G_LIKELY.
Remove setting caps on buffers explicitly, basesrc does that for us now.
Improve debug info.
Convert some non-fatal error into warnings.
Use g_ntohs for better portability.
Leave multicast groups when stopping.
When using external sockets, use getsockname() on them to fill up the
addr structure before calling methods that use the structure.
Should all fix #536903.
API: GstUDPSrc::auto-multicast property