timeline: reject mplayer2 EDL files, change EDL header

This was forgotten when the parser for mplayer2 EDL files was removed.

Change the header of the mpv EDL format to include a '#', so a naive
parser could skip the header as comment. (Maybe this is questionable;
on the other hand, if it can be simpler, why not.)

Also, strip the header in demux_edl.c before passing on the data, so the
header check doesn't need to be duplicated in tl_mpv_edl.c.
stream_lavf: fix a small memory leak

Also add an explanation why this special code path for rtsp is needed.
options: prefix options with "--" in help output

This is Better(tm).