maint: generate much of the THANKS file

maint: generate much of the THANKS file

Before this change, we had a tendency to manually list each
contributor's name in THANKS.  Now, each commit "Author" is
included in the generated THANKS file automatically, and most
of the old THANKS file is now a template, THANKS.in.
We'll still have to manually list the names of people who report
problems without a usable patch.

* THANKS.in: New file, derived from THANKS, but removing names of
those who are listed as git log 'Author:'s.
* THANKS: Remove file.
* thanks-gen: New file.
* Makefile.am (THANKS): New rule.
(EXTRA_DIST): Add .mailmap, THANKS.in and thanks-gen.
* .gitignore: Add THANKS and THANKS-to-translators.
* .mailmap: Unify on single address and name-spelling per contributor.