rtsp: Allow $ as interleaved packet indicator before a complete response header

Some RTSP servers ("HiIpcam/V100R003 VodServer/1.0.0") respond to
our keepalive GET_PARAMETER request by a truncated RTSP header
(lacking the final empty line to indicate a complete response
header). Prior to 764ec701, this worked just fine since we
reacted to the $ as interleaved packet indicator anywhere.

Since $ is a valid character within the response header lines,
764ec701 changed it to be ignored there. But to keep
compatibility with such broken servers, we need to at least
allow reacting to it at the start of lines.

Signed-off-by: Martin Storsjö <martin@martin.st>