mailmap: initial, trying to tidy up a lot of the commit history

This comes from the observation that a huge number of commit author
fields look quite strange (to my eyes), but quite often the
Signed-off-by: trailer has the correct name. For these I have updated
the name where it was obvious how to do so, however, I have not created
a mapping for the commit email to the Signed-off-by email, as whatever I
choose for email will become the prime candidate for inclusion in the
AUTHORS file, and care needs to be taken when acting without explicit
consent.

There's a small handful of commits that look like they were done on
local machines, or CI hosts, or similar, where the git authorship config
wasn't set up properly. Its obvious what this should look like, so I've
just done them.

The remainder is mapping Github noreply emails to either an
obviously-correct Signed-off-by trailer, or to a an author from another
commit. This was mostly done by hand, so there may be errors, but I
think its close. I do not understand where these come from - I know that
they're what commits made via Github web look like when there's no real
address set on the account, but I find it hard to believe that so many
of these came through the web, especially given the complexity of most
of the changes. I suspect there's some kind of merge helper tool in play
here. Regardless, the history is set now, and this tries to get it back
on track.

Obviously, all of this helps the history look tidy, but this also feeds
into the AUTHORS update script. See next commit.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #15374

.gitignore

@@ -45,6 +45,7 @@
 !.cirrus.yml
 !.gitignore
 !.gitmodules
+!.mailmap
 !AUTHORS
 !autogen.sh
 !CODE_OF_CONDUCT.md