mp3utils.c needs cleanup after tag-parsing merge

[sezero]

Now that mp3utils.c merged tag parsing as of commit fbf6f67729b (which I don't like), I dug my dirt pile and found a draft, a template maybe, of a clean-up attempt from Dec. 29, 2019: tags-clean-test-patch.txt

Doesn't apply cleanly, and probably doesn't compile, but it does have the general idea. In case anyone is interested (I don't.)

CC: @slouken, @Wohlstand

[Wohlstand]

A little note that I made several tweaks and updates with a time (to fix bugs or add missing functionality), I'll take a look, and if something is possible to apply and works fine, I'll send the pull request.

[sezero]

Remember that it is an incomplete draft, so it needs more places to touch in the file than it already does. Unfortunately, I couldn't find the mail traffic about it, but only the patch file itself.