Split rtsp_send_cmd() into two functions, one for the actual sending of the

Split rtsp_send_cmd() into two functions, one for the actual sending of the
command and a second, new function to read the reply to this command. This
will make it possible to read server notices that are not in response to a
command in future versions, such as EOS or interrupt notices. See "[PATCH]
rtsp.c: split rtsp_send_cmd() in a send- and a receive-function" thread.

Originally committed as revision 17797 to svn://svn.ffmpeg.org/ffmpeg/trunk