This aggregates the various authentication-results producing auth checks
into a more convenient function.
Part of this change improves some plumbing in the dkim checking (we no
longer raise an error for a missing From, but instead indicate a failed
dkim result), and we now support passing down a resolver name to the
various checking functions, to facilitate testing and other more
advanced use cases.
refs: https://github.com/KumoCorp/kumomta/issues/16
refs: https://github.com/KumoCorp/kumomta/issues/84
Wez Furlong