mail/rspamd: Backport upstream patch for reject-message with FreeBSD libmilter
There was a bug in upstream's (src/libserver/milter.c) compatibility with
FreeBSD's libmilter (contrib/sendmail/libmilter/engine.c).
The bug causes rspamd to send reject messages after set reply.

Reported by: Philip Jocks <pj@netzkommune.de>
Reviewed by: mat, vsevolod (maintainer)
Approved by: vsevolod (maintainer)
Sponsored by: Netzkommune GmbH
Differential Revision: https://reviews.freebsd.org/D14542