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.good

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+env > ../.data/post-data.out
+echo >> ../.data/post-data.out
+
+cat >> ../.data/post-data.out
+
+if [ "$RCPT_TO" == "blockme@testserver" ]; then
+	echo "¡No pasarán!"
+	exit 1
+fi
+
+echo "X-Post-Data: success"
+