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/eta.c b/eta.c
index 8e17d20..960bf4d 100644
--- a/eta.c
+++ b/eta.c
@@ -26,20 +26,20 @@
 			break;
 		case TD_RUNNING:
 			if (td_rw(td)) {
-				if (td->sequential)
-					c = 'M';
-				else
+				if (td_random(td))
 					c = 'm';
+				else
+					c = 'M';
 			} else if (td_read(td)) {
-				if (td->sequential)
-					c = 'R';
-				else
+				if (td_random(td))
 					c = 'r';
-			} else {
-				if (td->sequential)
-					c = 'W';
 				else
+					c = 'R';
+			} else {
+				if (td_random(td))
 					c = 'w';
+				else
+					c = 'W';
 			}
 			break;
 		case TD_VERIFYING: