patch 9.1.0664: MS-Windows: console vim did not switch back to main screen on exit

patch 9.1.0664: MS-Windows: console vim did not switch back to main screen on exit

Problem:  MS-Windows: console vim did not switch back to main screen on
          exit
Solution: switch back to main screen on exit (Ken Takata)

`vim.exe --cmd quit` quitted without switching back to the main screen
buffer and the alternate screen buffer was kept shown.

Restore the main screen buffer even when termcap mode has not been
started.

closes: #15443

Signed-off-by: Ken Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>