Supply a User-Agent header when opening rtsp streams.

Some rtsp servers like the IP Cam IcyBox IB-CAM2002 need it.
Fixes ticket #2761.
Reported, analyzed and tested by trac user imavra.
libavformat/mov.c: Skipped duplicated MOOV atom

 This should fix ticket 1378
 If we have parsed a moov atom, and found another one, just skip it.

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