rtsp: Accept options via private avoptions instead of URL options

rtsp: Accept options via private avoptions instead of URL options

Eventually, the old way of passing options by adding
stuff to the URL can be dropped.

This avoids having to tamper with the user-specified URL to
pass options on the transport mode. This also works better
with redirects, since the options don't need to be parsed out
from the URL.

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