avformat/udp: Allow to specify DSCP class

By appending `?dscp=26` to the URL, IP packets will be classified as
AF31 (assured forwarding for multimedia flows with low probability of
loss). On congested network, this allows a user to assign priorities to
flows.

Signed-off-by: Vincent Bernat <vincent@bernat.im>
avformat/rtpproto: Allow to specify DSCP class

By appending `?dscp=26` to the URL, IP packets will be classified as
AF31 (assured forwarding for multimedia flows with low probability of
loss). On congested network, this allows a user to assign priorities to
flows.

Signed-off-by: Vincent Bernat <vincent@bernat.im>