rtsp-stream: Listen on the multicast group for RTP/RTCP packets

When a UDP multicast transport is used it is expected that the server listens
for RTP and RTCP packets on the multicast group with the corresponding port.
Without this we will never get RTCP packets from clients in multicast mode.

https://bugzilla.gnome.org/show_bug.cgi?id=732238
Makefile: Add usage of build-checks step

Allows building checks without running them
signals: Fix copy-pasto in target-state signal offset