Implement librtmp seek support.

Implement flv_read_seek(), add a missing check on stream_index
and fix timestamp rounding in rtmp_read_seek().

Also add the flv_read_seek2() function, which is not enabled but is
useful as reference.
To actually implement flv_read_seek2() correctly, there would need to
be some corresponding av_url_read_fseek2() function to propagate the
timestamps down to the ByteIOContext and URLContext.

Patch by Howard Chu <hyc <at> highlandsun.com>.

See the thread:
Subject: [FFmpeg-devel] RTMP seek support
Date: 2010-03-28 23:35:02 GMT

Originally committed as revision 22904 to svn://svn.ffmpeg.org/ffmpeg/trunk
