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.

chasquid.go

@@ -62,6 +62,7 @@ func main() {
 	s := smtpsrv.NewServer()
 	s.Hostname = conf.Hostname
 	s.MaxDataSize = conf.MaxDataSizeMb * 1024 * 1024
+	s.PostDataHook = "hooks/post-data"
 
 	s.SetAliasesConfig(conf.SuffixSeparators, conf.DropCharacters)