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().

src/channel.c

@@ -2930,7 +2930,7 @@ append_to_buffer(buf_T *buffer, char_u *msg, channel_T *channel, ch_part_T part)
 		}
 	    }
 	}
-	redraw_buf_and_status_later(buffer, VALID);
+	redraw_buf_and_status_later(buffer, UPD_VALID);
 	channel_need_redraw = TRUE;
     }