make implicit "listen on socket" explicit, the default config no longer has

any implicit behavior
ok eric@, kn@

src/etc/mail/smtpd.conf

@@ -1,10 +1,12 @@
-#	$OpenBSD: smtpd.conf,v 1.13 2019/11/25 13:30:04 gilles Exp $
+#	$OpenBSD: smtpd.conf,v 1.14 2019/11/26 20:14:38 gilles Exp $
 
 # This is the smtpd server system-wide configuration file.
 # See smtpd.conf(5) for more information.
 
 table aliases file:/etc/mail/aliases
 
+listen on socket
+
 # To accept external mail, replace with: listen on all
 #
 listen on lo0