(check_timer): get rid of the DOS-specific menubar clock

(check_timer): get rid of the DOS-specific menubar clock
feature; call `display-time-filter' from time.el to display time
on the modeline instead.  This makes `display-time' work under DOS.
(abort): use our own function instead of the one from the library
which reverts the console device to cooked mode.
(syms_of_msdos): `dos-display-time'--a new variable for
communicating with `display-time'.