Maint: .mailmap: coalesce varying names/emails for the same personas

Main motivation was that I've discovered I mistakenly used my name sans
diacritics recently in commits authorship info, resulting in split
shortlog entries. So I went through all assumed splits like this,
unifying names and/or emails across the whole history. It's certainly
not perfect, take is as a first iteration open for fine tuning and
further additions.

Do raise your concerns, especially if that's you who is affected.