Remove access to rdt_data struct in functions called outside of the
DynamicProtocol* context. Doing so could lead to problems if we're accessing
Real servers serving non-RDT data (or the other way around). Temporarily,
this patch adds a _subscribe2() function which will soon be removed in one
of the subsequent commits. OK'ed by Luca in "Realmedia patch" thread on ML.
Originally committed as revision 15483 to svn://svn.ffmpeg.org/ffmpeg/trunk