Read a body from message_payload and accept data_sm (#84)

* Regression tests for a body in message_payload and for data_sm

* Read a body from message_payload and accept data_sm

* Assert the fixed message_payload and data_sm behaviour against Jasmin

* Record the message_payload and data_sm fixes in the Jasmin findings

* Read an inbound data_sm as the direction it travelled, and export messageOctets

* Refuse a segment with the code its stand-in command defines

* Name the stand-in the refusal status is read from

src/server.ts

@@ -241,6 +241,7 @@ function onConnection(sock: Socket, options: ServerOptions, server: SmppServer):
 		systemId: options.systemId ?? defaults.systemId,
 	});
 
+	session.linkEnd = 'smsc';
 	server.sessions.add(session);
 	session.on('close', () => server.sessions.delete(session));