Rename RTSPProtocol to RTSPLowerTransport, so that its name properly tells us

that it only describes the lower-level transport (TCP vs. UDP) and not the
actual data layout (e.g. RDT vs. RTP). See discussion in "Realmedia patch"
thread on ML.

Originally committed as revision 15481 to svn://svn.ffmpeg.org/ffmpeg/trunk