make notmuch cppcheck clean

[David Bremner]

I don't know if cppcheck is the best static checker there is, but it's
found a few problems already, and I had it installed, so this is an
initial response to Jani's suggestion of having one or more targets to
run static checkers.  I punted on a few of the tricker issues:
cppcheck is run serially here, even though it scales rather well, and
the target is optional. The rest of the series fixes complaints of
cppcheck so that in principle, we could run it automatically. My main
reason for not doing so is that I don't want builds that break with a
new release of cppcheck.