Implement RTSP/Realmedia-compatible OPTIONS command. See "Realmedia patch"

Implement RTSP/Realmedia-compatible OPTIONS command. See "Realmedia patch"
thread on mailinglist for discussion. This patch also implements a
RTSPServerType enum, which allows the RTSP to keep track of what kind of a
stream we're handling: standard-compliant RTP or a proprietary derivative.
This will be used in subsequent patches to implement more Realmedia-specific
extensions required to receive and parse data coming from a Realmedia server.

Originally committed as revision 15104 to svn://svn.ffmpeg.org/ffmpeg/trunk