playlist: don't add the base path twice

Loading a playlist with --playlist from a sub-directory added the
playlist's base path twice: one time in the playlist demuxer, and then
again in playlist_parse_file(). The latter function is used only for
--playlist, so it worked when loading the playlist directly.

(This is probably a mess-up when the MPlayer playlist parsers were
replaced with newer code.)

CC: @mpv-player/stable
player: don't allow remote playlists to load local files

Because that might be a bad idea.

Note that remote playlists still can use any protocol marked with
is_safe and is_network, because the case of http-hosted playlists
containing URLs using other streaming protocols is not unusual.