smtpsrv: Implement a post-DATA hook

This patch implements a post-DATA hook, which is run after receiving the
data but before sending a reply.

It can be used to implement content filtering when receiving email, for
example for passing the email through an anti-spam or an anti-virus.

test/t-10-hooks/config/hooks/post-data.bad4

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+echo $0 > ../.data/post-data.out
+
+echo -n "X-Post-DATA: valid header with no newline at the end"