decode-time now returns subsec too

decode-time now returns subsec too

The list that decode-time returns now contains an extra
trailing component that counts the subseconds part of the
original timestamp (Bug#36549).
This builds on a suggestion by Lars Ingebrigtsen in:
https://lists.gnu.org/r/emacs-devel/2019-07/msg00734.html
* doc/lispref/os.texi (Time Conversion):
* doc/misc/emacs-mime.texi (time-date):
* etc/NEWS: Document this.
* lisp/calendar/icalendar.el (icalendar--decode-isodatetime):
* lisp/calendar/iso8601.el (iso8601-parse)
(iso8601-parse-time, iso8601-parse-duration)
(iso8601--decoded-time):
* lisp/calendar/parse-time.el (parse-time-string):
* lisp/calendar/time-date.el (make-decoded-time)
(decoded-time-set-defaults):
* lisp/org/org.el (org-fix-decoded-time)
(org-parse-time-string):
* src/timefns.c (Fdecode_time):
Generate subsec member for decoded time.
* lisp/calendar/time-date.el (decoded-time-add)
Add the decoded subsec too.
* lisp/simple.el (decoded-time): New subsec member.
* src/data.c (Frem): Simplify zero-check to match that of new Fmod.
(integer_mod): New function, with most of the guts of the old Fmod.
Remove redundant zero-check.
(Fmod): Use it.
* src/timefns.c (Fencode_time): Handle new subsec member
or (with the obsolescent calling convention) subsec arg.
It defaults to 0.
* test/lisp/calendar/icalendar-tests.el:
(icalendar--decode-isodatetime):
* test/lisp/calendar/iso8601-tests.el (test-iso8601-date-years)
(test-iso8601-date-dates, test-iso8601-date-obsolete)
(test-iso8601-date-weeks, test-iso8601-date-ordinals)
(test-iso8601-time, test-iso8601-combined)
(test-iso8601-duration, test-iso8601-intervals)
(standard-test-dates, standard-test-time-of-day-fractions)
(standard-test-time-of-day-beginning-of-day)
(standard-test-time-of-day-utc)
(standard-test-time-of-day-zone)
(standard-test-date-and-time-of-day, standard-test-interval):
* test/lisp/calendar/parse-time-tests.el (parse-time-tests):
* test/src/timefns-tests.el (format-time-string-with-zone)
(encode-time-dst-numeric-zone):
Adjust to match new behavior.