Streamline thread_data data direction setting and checking

Currently it's a mess of ->ddir, ->iomix and ->sequential. Add
a TD_DDIR_* for each of these, so we can store them as one value.

A prerequisite for the next parsing cleanup.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/engines/net.c b/engines/net.c
index 55b2128..60a6877 100644
--- a/engines/net.c
+++ b/engines/net.c
@@ -223,7 +223,7 @@
 		return 1;
 	}
 
-	if (td->iomix) {
+	if (td_rw(td)) {
 		log_err("fio: network connections must be read OR write\n");
 		return 1;
 	}
@@ -241,7 +241,7 @@
 	strcpy(host, buf);
 	port = atoi(sep);
 
-	if (td->ddir == DDIR_READ) {
+	if (td_read(td)) {
 		send_to_net(td) = 0;
 		ret = fio_netio_setup_listen(td, port);
 	} else {