patch 9.0.0100: get hit-enter prompt for system() when '!' is in 'guioptions'

Problem:    Get hit-enter prompt for system() when '!' is in 'guioptions'.
Solution:   Do not call wait_return() when not redrawing. (closes #3327)
diff --git a/src/os_unix.c b/src/os_unix.c
index 18c766d..07762f4 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -4522,7 +4522,10 @@
     // restore curwin/curbuf and a few other things
     aucmd_restbuf(&aco);
 
-    wait_return(TRUE);
+    // only require pressing Enter when redrawing, to avoid that system() gets
+    // the hit-enter prompt even though it didn't output anything.
+    if (!RedrawingDisabled)
+	wait_return(TRUE);
     do_buffer(DOBUF_WIPE, DOBUF_FIRST, FORWARD, buf->b_fnum, TRUE);
 
 theend: