Cygwin: console: Add a workaround for "ESC 7" and "ESC 8".

- In xterm compatible mode, "ESC 7" and "ESC 8" do not work properly
  in the senario:
   1) Execute /bin/ls /bin to fill screen.
   2) Sned CSI?1049h to alternate screen.
   3) Reduce window size.
   4) Send CSI?1049l to resume screen.
   5) Send "ESC 7" and "ESC 8".
  After sending "ESC 8", the cursor goes to incorrect position. This
  patch adds a workaround for this issue.

winsup/cygwin/fhandler.h

@@ -1869,6 +1869,7 @@ class dev_console
   bool alternate_charset_active;
   bool metabit;
   char backspace_keycode;
+  bool screen_alternated; /* For xterm compatible mode only */
 
   char my_title_buf [TITLESIZE + 1];