Add Static Code Analyzer "cppcheck" as make target

The MR adds the static code analyzer "cppcheck" as optional make target and implements a few of the suggested fixes.

cppcheck reports quite some false positives but a few things have been caught correctly.

Also updates the manual accordingly, describing the new make targets.

There is no intention of using this in the CI, this is more meant as commodity for occasional manual checks.

@kwolters this is ready for review.