Apparently mpv supports loading config files from the same directory as
the mpv.exe. This is a fallback of some sort. It used the old_home
mechanism.
I want to add a warning if old_home exists, but that would always show
the warning on win32. Obviously we don't want that.
Add a separate exe_dir entry to deal with that.
Untested, but probably works.
Mistakenly reverted as part of the default configuration directory
location switch-back in aa18a8e1cd.
Separation of the mpv executable directory from old_path is a
good change now that we warn about the old_config directory also
existing.
Fixes#8232Fixes#8244Fixes#8262