[Fix] Aliases: refuse to rewrite From domains

[Fix] Aliases: refuse to rewrite From domains

The From address is the sender's identity: SPF, DKIM and DMARC all key
on its domain. Service rules and alias resolution (e.g. a virtual alias
mapping to another domain) may only touch the local part of the From
address; any rewrite that would change the domain is now discarded.
Recipient rewriting is unaffected: recipients are interpreted by the
receiving host, and no authentication protocol keys on them.

Also fix a spurious task:set_from call: the set_from decision used the
callback-wide alias_resolved flag, so a rewrite of any earlier address
(e.g. an envelope recipient) caused the unmodified MIME From to be
marked as aliased and duplicated.

Issue: #6137