rtsp: Only interpret $ as interleaved packet indicator at the start of replies

Allow $ as character anywhere within normal RTSP replies - both
within the lines, and as the first character of RTSP header lines.
(The existing old comment indicated that an inline packet could
start at any line within a RTSP reply header, but that doesn't
sound valid to me, and I'm not sure if the existing code
handled that correctly either.)

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>