Remove some assumptions about timestamp format

Remove some assumptions about timestamp format

These changes remove some assumptions of Lisp code on timestamp
format.  Although we’re not going to change the default format any
time soon, I went looking for code that was too intimate about
details of timestamp format and removed assumptions where this was
easy to do with current Emacs primitives.
* lisp/ido.el (ido-wash-history):
Fix test for zero timestamp.
* lisp/time.el (display-time-event-handler):
Use time-less-p rather than doing it by hand.
(display-time-update): Simplify by using float-time
instead of doing the equivalent by hand.
* lisp/url/url-auth.el (url-digest-auth-make-cnonce):
* test/lisp/calendar/parse-time-tests.el (parse-time-tests):
* test/lisp/emacs-lisp/timer-tests.el (timer-test-multiple-of-time):
* test/lisp/net/tramp-tests.el:
(tramp-test19-directory-files-and-attributes)
(tramp-test22-file-times, tramp-test23-visited-file-modtime):
Don’t assume detailed format of returned Lisp timestamps.