avformat/rtpproto: when local rtp port and local rtcp port are not specified,...

avformat/rtpproto: when local rtp port and local rtcp port are not specified, try to make two successive ports and retry 3 times when failed

some video players on Android will not send udp hole punching messages if the rtcp port and rtp port are not two successive integers.
So, if the video player is behind NAT, it could not receive and rtp messages via udp

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>