Enable av_read_pause(), av_read_play() and the ASF demuxer's av_read_seek()

to use the protocol-native functionality if available.
Patch by Björn Axelsson: bjorn point axelsson at intinor dot se
Original thread: [FFmpeg-devel] [PATCH][4/4] Enable use of the extended API
Date: Thu Nov 22 16:01:06 CET 2007

Originally committed as revision 11248 to svn://svn.ffmpeg.org/ffmpeg/trunk