[tlbuild] tuneup21 from Don Knuth

[Karl Berry]

Don has finished (he hopes) the new 2021 tuneup versions of TeX, MF, and
friends, after a couple rounds of "internal" bug fixes from (mostly)
DRF. He has asked that we ("implementors") give it some testing before
it is released more publicly, e.g., uploading to CTAN.

So, I've committed the new .web files to TL, along with updated .ch
files (thanks to Akira and Hironori for dealing with the engines, i.e.,
the hard part) -- r57558. The build works for me, and trip/trap/other
tests pass, but I haven't done much more as yet. LuaTeX and siblings are
not updated since they are not Pascal-based; Luigi, I trust you will do
the needful.

Don also made changes to plain.tex and a few other runtime files.
Unfortunately they are not so easy to test since it would be wrong to
update them in the real TL runtime and thus push them out to all TL
users right now. So I'm only including those changes in the diff
attached. (BTW, the file plain/base/letter.tex was renamed to
letterformat.tex; no other filenames were changed.)  I could make a TL
branch with the changes if it would be useful now; else I'll commit them
after we start the TL pretest.

A second set of diffs is for the source (.web etc.) files, just so they
are recorded in the archive here.

Hope the TL builds work out ok. -k

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