Support broadcast destination for udp protocol

Use the required socket option SO_BROADCAST to be able to stream to a broadcast
address.

Prior to the patch, trying to stream to a broadcast address was resulting to the
following error:

av_interleaved_write_frame(): Permission denied

The patch has been tested with:

ffmpeg -f v4l2 -framerate 30 -input_format yuyv422 -video_size 640x480 -i /dev/video0 \
 -c:v libx264 -profile:v high -preset ultrafast -tune zerolatency -b:v 500k -pix_fmt yuv420p \
 -f mpegts udp://192.168.1.255:5004?broadcast=1

I have added an option to let the user explicitly request broadcast in order to avoid
ffmpeg to broadcast unintentionally.

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>