player: properly resume playlists loaded at runtime

If a playlist is loaded from the internal playlist (like
"mpv playlist.m3u"), then attempt to resume from it.

CC: @mpv-player/stable
player: add --media-title option

Requested by ChrisK2.
demux: allow increasing filepos only

The last demuxed file position (demuxer->filepos) is used to estimate
the total playback percentage in files with possible timestamp resets
(like MPEG-PS). Until know, reading from any stream set this position
freely. This makes the position jump around.

Fix this by allowing icnreasing file position only. Reset it on seeking.
With crazy formats, this still could go wrong, but there's only so much
you can do.
path: remove XDG bullshit

Don't worry, your ~/.config/... paths are safe. This merely removes
handling of $XDG_CONFIG_DIRS for global paths.

Maybe there is a better solution for this, like still including the
"traditional" config dir. But I will leave the fine reading of this
(crappy) spec and fixing the code accordingly to someone else. So, if
anyone has interest in getting this behavior back, you will have to
write a patch. This patch should _also_ not break expected behavior.

Fixes #1060.