Fix dmarc reject (#260)
Fix dmarc reject (#260)

* 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 the dmarc_reject note on the transaction
It's probably possible to have one valid email after an invalid one using the same connection (even if not common)

smtpd/qpsmtpd,smtpd/qpsmtpd,priyadi/qpsmtpd,salvis/qpsmtpd,salvis/qpsmtpd,priyadi/qpsmtpd,priyadi/qpsmtpd