bug#35383: 27.0.50; Complete process of decoding Gnus group names

[Eric Abrahamsen]

By god, I think this branch (scratch/gnus-decoded) is done. I've been
running it for several weeks, have added some additional fixes, and have
merged a few times from master to keep up with development.

I just rebased onto master (it went better than I expected, the
merge-from-master commits were ignored), so now I have a local branch
with 19 commits in it, some of which are just WIP commits, others small
fixes and tweaks to previous commits in the branch.

If anyone wants to see this again I can push a new branch (say
feature/gnus-decoded) with either squashed or unsquashed commits, or I
can post patches here.

I'm assuming the final branch should be squashed. One thing about the
code here is that it preserves backwards compatibility with Gnus' files
(newsrc.eld, the registry persistence file if that's used, and the agent
category files), keeping non-ascii group names written in an encoded
format. My idea was that, as Emacs 27 is released, we could bump the
Gnus version a point, which would trigger `gnus-convert-old-newsrc' (or
maybe this would be a clean?), and the permanent decoding could be done
there -- the user would at least be warned that their files might not be
backwards compatible.

Anyway, if anyone has any opinions on any of this, let me know!

Eric