transports: fix the tests a bit for closed, to be clearer

Also increase the backoff times to 1/2 seconds, if we are out of
files or memory.  If someone is up against this, they want to let
the system calm down.

src/sp/transport/ipc/ipc.c

@@ -728,7 +728,7 @@ ipc_ep_accept_cb(void *arg)
 		break;
 	case NNG_ENOMEM:
 	case NNG_ENOFILES:
-		nng_sleep_aio(10, &ep->time_aio);
+		nng_sleep_aio(500, &ep->time_aio);
 		break;
 
 	default:

src/sp/transport/socket/sockfd.c

@@ -687,7 +687,7 @@ sfd_tran_accept_cb(void *arg)
 		ep->useraio = NULL;
 		nni_aio_finish_error(aio, rv);
 	}
-	if (!ep->closed) {
+	if (rv != NNG_ECLOSED) {
 		nng_stream_listener_accept(ep->listener, &ep->connaio);
 	}
 	nni_mtx_unlock(&ep->mtx);

src/sp/transport/tcp/tcp.c

@@ -716,15 +716,15 @@ tcptran_accept_cb(void *arg)
 	}
 	switch (rv) {
 
+	case NNG_ECLOSED:
+		break;
 	case NNG_ENOMEM:
 	case NNG_ENOFILES:
-		nng_sleep_aio(10, &ep->timeaio);
+		nng_sleep_aio(500, &ep->timeaio);
 		break;
 
 	default:
-		if (!ep->closed) {
-			nng_stream_listener_accept(ep->listener, &ep->connaio);
-		}
+		nng_stream_listener_accept(ep->listener, &ep->connaio);
 		break;
 	}
 	nni_mtx_unlock(&ep->mtx);

src/sp/transport/tls/tls.c

@@ -682,19 +682,19 @@ tlstran_accept_cb(void *arg)
 	}
 	switch (rv) {
 
+	case NNG_ECLOSED:
+		break;
 	case NNG_ENOMEM:
 	case NNG_ENOFILES:
 		// We need to cool down here, to avoid spinning.
-		nng_sleep_aio(10, &ep->timeaio);
+		nng_sleep_aio(500, &ep->timeaio);
 		break;
 
 	default:
 		// Start another accept. This is done because we want to
 		// ensure that TLS negotiations are disconnected from
 		// the upper layer accept logic.
-		if (!ep->closed) {
-			nng_stream_listener_accept(ep->listener, &ep->connaio);
-		}
+		nng_stream_listener_accept(ep->listener, &ep->connaio);
 		break;
 	}
 	nni_mtx_unlock(&ep->mtx);