player: don't decrement --loop-file=N and add remaining-file-loops

This stops decreasing numerical values of --loop-file on each iteration
so that loop-file=N loops every playlist entry without having to add
--loop-file to --reset-on-next-file.

The current behavior confuses users as seen in:

https://github.com/mpv-player/mpv/issues/2481
https://github.com/mpv-player/mpv/issues/5943
https://github.com/mpv-player/mpv/issues/11291
https://github.com/mpv-player/mpv/issues/13860
https://www.reddit.com/r/mpv/comments/rcwnrw/looping_each_file_n_times_in_a_playlist/

Also options are supposed to reflect the value configured by the user
and not change on their own.

A remaining-file-loops property is exposed as a replacement to check how
many loops are left.

DOCS/interface-changes/loop.txt

@@ -0,0 +1,2 @@
+numerical values of `--loop-file` no longer decrease on each iteration
+add `remaining-file-loops` property as a replacement to get the remaining loop count