awk: Reduce diffs with upstream to almost nothing.

In the merge of 20210215, I left two merge conflicts #if 0'd by mistake
to check later rather than resolve them as part of the merge. This code
turns out to be from the original one-true-awk import and not FreeBSD
specific, so remove them.

Remove a extra definition of HAT.

Remove a stylistic change that also appears to be a mismerge along the
way.

Remove FREEBSD-upgrade. Nobody has updated it since the original 2007
cvs import. It talks about old CVS branches that never made it into svn,
let alone git. New imports will follow the standard practices now, so
there's nothing left to document.

Move README to README.md and copy the README.md from upstream over.

This leaves just the $FreeBSD$ lines (which remain for the stable/12
merge) and the strcoll part of ru@'s r201989/d98dd8e5f94c as the only
diffs with upstream. FreeBSD also still has its own man page, which I
don't plan on changing. Once this commit is merged to stable/12, I plan
no further merges to stable/12. Sometime after that I'll remove the
$FreeBSD$ lines to reduce the diffs even more (though i want to make
sure plans won't change first). I also plan to talk to upstream about
this change...

MFC After: 2 weeks
Sponsored by: Netflix

(cherry picked from commit f68a53dba933f9c1143863603f5a16fdbe5f070e)