Add SignHeaders From

[seriv]

From the man opendkim.conf:
OversignHeaders (dataset)
Specifies a set of header fields that should be included
in all signature header lists (the "h=" tag) once more than
the number of times they were actually present in the signed
message. The set is empty by default.
The purpose of this, and especially of listing an absent
header field, is to prevent the addition of important fields
between the signer and the verifier. Since the verifier would
include that header field when performing verification
if it had been added by an intermediary, the signed message
and the verified message were different and the verification
would fail. Note that listing a field name here and not
listing it in the SignHeaders list is likely to generate
invalid signatures.

[lvicainne]

Thank you for this contribution

[Dan33l]

This PR looks to be a breaking change.

Opendkim manual says :

SignHeaders: Specifies the set of header fields that should be included when generating signatures. If the list omits any header field that is mandated by the DKIM specification, those fields are implicitly added. By default, those fields listed in the DKIM specification as "SHOULD" be signed (RFC6376, Section 5.4) will be signed by the filter.

https://datatracker.ietf.org/doc/html/rfc6376#section-5.4.1

Before this commit mails from my MTA have h=Subject:To:From:Date, after only h=From