patch 9.0.0206: redraw flags are not named specifically

Problem:    Redraw flags are not named specifically.
Solution:   Prefix "UPD_" to the flags, for UPDate_screen().
diff --git a/src/channel.c b/src/channel.c
index 8a64d17..0bfeeac 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -2930,7 +2930,7 @@
 		}
 	    }
 	}
-	redraw_buf_and_status_later(buffer, VALID);
+	redraw_buf_and_status_later(buffer, UPD_VALID);
 	channel_need_redraw = TRUE;
     }