Implement a new command-line parser, based on CLI11.
It's a bit more C++'ish, and allows for a lot of fancy things... some of
which we have implemented here.
Update the various commands to use the new Options struct
Remove the old help strings; instead e.g. `mu help view` opens the
manpage.
Integrate the guile scripts more tightly.
The 'processed' statistic for indexing was more-or-less synonymous for
'updated'; let's change to something more useful, 'checked' which roughly means
the number of messages checked for updates (typically a cheap timestamp check).
Update all cc code using .clang-format; please do so as well for future PRs
etc.; emacs has a handy 'clang-format' mode to make this automatic.
For comparing old changes with git blame, we can disregard this one using
--ignore-rev
(see https://www.moxio.com/blog/43/ignoring-bulk-change-commits-with-git-blame )
Some #includes were missing for the latter (but only noticeable on some
systems - e.g., build breaks on Cygwin).
So let's replace with something that works equally everywhere.
Fixes: #2060