Support for Japanese display on DOS/V systems.

Support for Japanese display on DOS/V systems.

(screen_old_address, screen_virtual_segment,
screen_virtual_offset): New variables.
(dosv_refresh_virtual_screen): New function.
(dos_direct_output, dos_set_window_size, IT_write_glyphs,
IT_clear_end_of_line, IT_clear_screen, IT_display_cursor,
IT_reset_terminal_modes, XMenuActivate, abort):  Call
dosv_refresh_virtual_screen if under DOS/V.
(IT_set_terminal_modes): If under DOS/V, update the address of
primary screen buffer.
(internal_terminal_init): Zero out screen_old_address, in case
Emacs was dumped under DOS/V.
(dos_get_saved_screen): Return failure indication if no screen was
saved.