ARC plugin: re-use arc= results from AuthRes plugin when available

The ARC plugin now checks for parsed arc= results from the AuthRes
plugin before performing its own Mail::DKIM::ARC::Verifier-based
cryptographic verification.  Plugin priorities swapped so AuthRes (-20)
runs before ARC (-10).

Trusted AAR parsing refactored into a shared helper so it works both
with the native verifier path and the AuthRes path (which parses
ARC-Seal headers directly for trusted domain detection).

If AuthRes is not loaded, behavior is unchanged.

git-svn-id: https://svn.apache.org/repos/asf/spamassassin/trunk@1932360 13f79535-47bb-0310-9956-ffa450edef68