feat: add NoReportsList to suppress report delivery to known-bad destinations

[thegushi]

Closes #319.

What it does

Adds a NoReportsList config option pointing to a file of email addresses and/or domains to which reports should never be delivered, regardless of what the DMARC policy record requests.

Use case: some rua= and ruf= destinations are persistently dead, rate-limited, or otherwise unable to accept DMARC reports (e.g. a Gmail inbox published as a reporting address for a high-volume sender). There is currently no way to suppress delivery to these destinations short of modifying the database or source code.

File format

One entry per line. Lines beginning with # are comments. An entry may be:

• A full email address: reports@gmail.com
• A bare domain: gmail.com (suppresses all addresses at that domain)

Matching is case-insensitive.

Where it applies

• Milter (opendmarc): failure report (ruf=) delivery is suppressed before the report is constructed
• opendmarc-reports: aggregate report (rua=) delivery is suppressed before SMTP delivery, alongside the existing stale_skip and db_skip checks

The same file path is used for both, configured via a single NoReportsList directive in opendmarc.conf (or --no-reports-list on the opendmarc-reports command line).

Relationship to IgnoreMailTo (PR #393)

IgnoreMailTo prevents inbound messages to a reporting address from being recorded in the history file — it breaks loops at the source. NoReportsList prevents outbound report delivery to known-bad destinations. Different problem, different fix.

[stoecker]

##  NoReportsList path
##  	default (none)
##
##  Specifies a file containing email addresses and/or domains to which
##  failure reports (ruf=) should never be delivered.  Each line is one
##  entry; lines beginning with # are comments.  An entry may be a full
##  address (reports@gmail.com) or a bare domain (gmail.com) to suppress
##  all addresses at that domain.  The same file is honoured by
##  opendmarc-reports for aggregate report delivery suppression.

That's unclear.

Domain is xyz.com, ruf=hallo@abc.com

When using the bare domain, is xyz.com correct? The text sounds like abc.com would be the correct choice, but that's something different as it blocks all domains using the same mail domain.