patch 8.0.0813: cannot use a terminal window while the job is running

Problem:    Cannot use Vim commands in a terminal window while the job is
            running.
Solution:   Implement Terminal Normal mode.
diff --git a/src/main.c b/src/main.c
index 268c256..81eb1a6 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1356,11 +1356,17 @@
 	else
 	{
 #ifdef FEAT_TERMINAL
-	    if (curbuf->b_term != NULL && oa.op_type == OP_NOP
-							  && oa.regname == NUL)
-		terminal_loop();
+	    if (term_use_loop() && oa.op_type == OP_NOP && oa.regname == NUL)
+	    {
+		/* If terminal_loop() returns OK we got a key that is handled
+		 * in Normal model.  With FAIL the terminal was closed and the
+		 * screen needs to be redrawn. */
+		if (terminal_loop() == OK)
+		    normal_cmd(&oa, TRUE);
+	    }
+	    else
 #endif
-	    normal_cmd(&oa, TRUE);
+		normal_cmd(&oa, TRUE);
 	}
     }
 }