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];
 

winsup/cygwin/fhandler_console.cc

@@ -207,6 +207,8 @@ fhandler_console::setup ()
       con.dwLastCursorPosition.Y = -1;
       con.dwLastMousePosition.X = -1;
       con.dwLastMousePosition.Y = -1;
+      con.savex = con.savey = -1;
+      con.screen_alternated = false;
       con.dwLastButtonState = 0;	/* none pressed */
       con.last_button_code = 3;	/* released */
       con.underline_color = FOREGROUND_GREEN | FOREGROUND_BLUE;
@@ -2130,6 +2132,10 @@ fhandler_console::char_command (char c)
 	  break;
 	case 'h': /* DECSET */
 	case 'l': /* DECRST */
+	  if (c == 'h')
+	    con.screen_alternated = true;
+	  else
+	    con.screen_alternated = false;
 	  wpbuf_put (c);
 	  /* Just send the sequence */
 	  WriteConsoleA (get_output_handle (), wpbuf, wpixput, &wn, 0);
@@ -2989,6 +2995,36 @@ fhandler_console::write (const void *vsrc, size_t len)
 	      con.saw_space = false;
 	      con.saw_exclamation_mark = false;
 	    }
+	  else if (*src == '8')		/* DECRC Restore cursor position */
+	    {
+	      if (con.screen_alternated)
+		{
+		  /* For xterm mode only */
+		  DWORD n;
+		  /* Just send the sequence */
+		  wpbuf_put (*src);
+		  WriteConsoleA (get_output_handle (), wpbuf, wpixput, &n, 0);
+		}
+	      else if (con.savex >= 0 && con.savey >= 0)
+		cursor_set (false, con.savex, con.savey);
+	      con.state = normal;
+	      wpixput = 0;
+	    }
+	  else if (*src == '7')		/* DECSC Save cursor position */
+	    {
+	      if (con.screen_alternated)
+		{
+		  /* For xterm mode only */
+		  DWORD n;
+		  /* Just send the sequence */
+		  wpbuf_put (*src);
+		  WriteConsoleA (get_output_handle (), wpbuf, wpixput, &n, 0);
+		}
+	      else
+		cursor_get (&con.savex, &con.savey);
+	      con.state = normal;
+	      wpixput = 0;
+	    }
 	  else if (wincap.has_con_24bit_colors () && !con_is_legacy
 		   && wincap.has_con_broken_il_dl () && *src == 'M')
 	    { /* Reverse Index (scroll down) */
@@ -3019,12 +3055,15 @@ fhandler_console::write (const void *vsrc, size_t len)
 	      wpixput = 0;
 	    }
 	  else if (wincap.has_con_24bit_colors () && !con_is_legacy)
-	    { /* Only CSI is handled in xterm compatible mode. */
+	    {
 	      if (*src == 'c') /* RIS Full reset */
 		{
 		  con.scroll_region.Top = 0;
 		  con.scroll_region.Bottom = -1;
 		}
+	      /* ESC sequences below (e.g. OSC, etc) are left to xterm
+		 emulation in xterm compatible mode, therefore, are not
+		 handled and just sent them. */
 	      wpbuf_put (*src);
 	      /* Just send the sequence */
 	      DWORD n;
@@ -3067,18 +3106,6 @@ fhandler_console::write (const void *vsrc, size_t len)
 	      con.state = normal;
 	      wpixput = 0;
 	    }
-	  else if (*src == '8')		/* DECRC Restore cursor position */
-	    {
-	      cursor_set (false, con.savex, con.savey);
-	      con.state = normal;
-	      wpixput = 0;
-	    }
-	  else if (*src == '7')		/* DECSC Save cursor position */
-	    {
-	      cursor_get (&con.savex, &con.savey);
-	      con.state = normal;
-	      wpixput = 0;
-	    }
 	  else if (*src == 'R')		/* ? */
 	    {
 	      con.state = normal;