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/msmtprc

@@ -0,0 +1,14 @@
+account default
+
+host testserver
+port 1587
+
+tls on
+tls_trust_file config/certs/testserver/fullchain.pem
+
+from user@testserver
+
+auth on
+user user@testserver
+password secretpassword
+