Dmarc result note (#261)

* Update data_post_headers documentation
We cannot reject at this stage, which is only there to alter headers.
Fix #258

* DMARC plugin: reject in data_post
Followup of #258: we cannot reject a connection during data_post_headers. So add a new hook in data_post to do the real rejection

* Add a note with dmarc_result
So other plugins can easily check DMARC status.
For example, a simple plugin can be added after DMARC when SPF hard failed, but no DMARC policy is published (so no rejection by the DMARC plugin).
With this note, we can easily check the result of SPF and DMARC and reject if the first is failed, and the second is none

plugins/dmarc

@@ -154,6 +154,7 @@ sub check_dmarc {
         };
     };
 
+    $transaction->notes('dmarc_result', $dmarc->result);
     my $disposition = $dmarc->result->disposition;
     my $result = $dmarc->result->result;
     my $auth_str = "dmarc=$result";