Minor improvements

Including escaping/quoting fixes. I also want to note that the escape method currently used ([player/configfiles.c:318](https://github.com/mpv-player/mpv/blob/7c07da57e3797b26c6e896bd27afc2ed7d639a18/player/configfiles.c#L318)) would fail in case of backslashes' (they would be expanded) or newlines' occurrence inside strings to be quoted. None of the current `backup_properties` could contain them, though.