rtmpproto: Support alternative slist parameter in rtmp URLs

Support the URL scheme where the playpath is in an RTMP URL is
passed as the slist argument and the app is given infront of the
query part of the URL:

rtmp://host[:port]/[app]?slist=[playpath]

(other arguments in the query part are stripped as they are not used)

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