Add SPF/DNSBL/SpamAssassin/ClamAV headers to email

[rrendec]

Add SPF/DNSBL/SpamAssassin/ClamAV as X-Mailfilter-* headers to the email. This makes it much easier to diagnose unexpected email classification because scan results can be found inside the email and it's not necessary to dig up the logs.

Proposal:

• X-Mailfilter-SPF - use Spf.resultStrMap[rsp.result]
• X-Mailfilter-DNSBL - use a similar value as the Sys.log() call; add multiple headers (one for each DNSBL)
• X-Mailfilter-SpamAssassin-Status - clean, spam or skipped (e.g. due to message size)
• X-Mailfilter-SpamAssassin-Score - include score, required and bayes e.g. 2.7/5.0; 0.5
• X-Mailfilter-SpamAssassin-Version - include version, subversion, rulesversion e.g. 3.4.6/2021-04-09/1923544
• X-Mailfilter-ClamAV-Status - clean, infected; <virus_id> or skipped (e.g. due to message size)
• X-Mailfilter-ClamAV-Version - include progver, dbver, dbdate e.g. ClamAV 1.0.7/27388/Wed Sep 4 04:40:46 2024