Draft: Add functions to evaluate signature for specific UserID, only

Thomas Friedrichsmeierrequested to merge
work/tfry/sender_signature into master

In the context of email, what matters is not the most trusted UserID for a given signature, but a UserID that actually matches the purported sender. So far the sender was only taken into account as a formatting hint.

This also exports two new helper functions, which can be used in mail clients to detect a mismatch between sender and most trusted UserID.

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See mimetreeparser!95 (closed) .

Edited by Thomas Friedrichsmeier

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