mail: mbox handling as a part of mail handling, refactored from patchbomb

patchbomb: minor refactoring of mbox functionality, preparing for move This allows reuse of the mbox functionality but causes minor changes in the patchbomb console output when writing to a mbox.

tests: run check-code with warnings and maintain a whitelist check-code has fine warning checks, but they are a bit noisy and nobody used them. Now the warnings will be run in the test suite, where a list of accepted warnings will be maintained. Those who introduce or touch a line with a warning will now have to update the whitelist ... or fix the warning.