s2smanager: Make require_s2s_encryption do what it says on the tin

core/s2smanager.lua

@@ -453,6 +453,16 @@ function verify_dialback(id, to, from, key)
 end
 
 function make_authenticated(session, host)
+	if not session.secure then
+		local local_host = session.direction == "incoming" and session.to_host or session.from_host;
+		if config.get(local_host, "core", "require_s2s_encryption")) then
+			session:close({
+				condition = "policy-violation",
+				text = "Encrypted server-to-server communication is required but was not "
+				       ..((session.direction == "outgoing" and "offered") or "used")
+			});
+		end
+	end
 	if session.type == "s2sout_unauthed" then
 		session.type = "s2sout";
 	elseif session.type == "s2sin_unauthed" then