player: always load playlists

Until now, you had to use --load-unsafe-playlists or --playlist to get
playlists loaded. Change this and always load playlists by default.

This still attempts to reject unsafe URLs. For example, trying to invoke
libavdevice pseudo-demuxer is explicitly prevented. Local paths and any
http links (and some more) are always allowed.
client API: fix memory leak with rejected events

The event was copied early, and wasn't released if it was rejected
instead of being added to the event queue. Fix by copying the event at a
point when it's certainly added to the event queue.

The dup_event_data() function is merely moved.
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.
man: fix a whole bunch of typos