Add FIXMEs for subsecond support

Add FIXMEs for subsecond support

This adds FIXMEs to areas where Lisp code should support
subsecond information in broken-down timestamps.
It also fixes some unnecessary truncation of timestamps, and
ports the code to a hypothetical future Emacs version where
(decode-time) returns subsecond timestamps by default.
* lisp/calc/calc-forms.el (calc-time, math-iso-dt-to-date)
(calcFunc-now):
* lisp/calendar/icalendar.el (icalendar--add-decoded-times):
* lisp/calendar/iso8601.el (iso8601-parse-interval):
Truncate seconds to an integer, and add a FIXME about
subseconds support.
* lisp/calendar/icalendar.el (icalendar--decode-isodatetime)
(icalendar--decode-isoduration):
Add a FIXME about subseconds support.
* lisp/gnus/gnus-delay.el (gnus-delay-article):
Don’t truncate seconds to an integer, as there’s no need
to do that here.
* lisp/gnus/gnus-util.el (gnus-seconds-today)
(gnus-seconds-month, gnus-seconds-year):
* lisp/gnus/message.el (message-make-expires-date):
* lisp/org/org-timer.el (org-timer-show-remaining-time):
* lisp/vc/ediff-mult.el (ediff-format-date):
Truncate seconds to an integer, as that’s what’s wanted here.
* lisp/midnight.el (midnight-next):
Ceiling seconds to an integer, as that’s what wanted here.