fix #2971: DKIM: add setting to use From header when signing
Following RFC 5322 [2], we add an option to use the address from the
`From:` header instead of the Envelope From address.
From RFC 7489 [1]:
To illustrate, in relaxed mode, if a validated DKIM signature
successfully verifies with a "d=" domain of "example.com", and the
RFC5322.From address is "alerts@news.example.com", the DKIM "d="
domain and the RFC5322.From domain are considered to be "in
alignment". In strict mode, this test would fail, since the "d="
domain does not exactly match the FQDN of the address.