player: change cover-art-auto behavior

This makes cover-art-auto behave more like sub-auto and audio-file-auto:

- load cover art with a language, e.g. if playing foo.mp3, foo.en.jpg
will be loaded with lang=en
- load cover art containing the media filename with fuzzy and all, e.g.
'foo (large).jpg'
- make all/2 load all images in the directory, and make fuzzy/1 the
default

These are all uncommon use cases, but synchronizing the behavior of the
external file options simplifies the code.

options/options.c

@@ -1018,7 +1018,7 @@ static const struct MPOpts mp_default_opts = {
     .pitch_correction = 1,
     .sub_auto = 0,
     .audiofile_auto = -1,
-    .coverart_auto = 2,
+    .coverart_auto = 1,
     .osd_bar_visible = 1,
     .screenshot_template = "mpv-shot%n",
     .play_dir = 1,