time-equal, and time values of infinity and NaN

time-equal, and time values of infinity and NaN

* doc/lispref/os.texi (Time Calculations):
Document time-equal, and the behavior on NaNs and infinities of
time-less-p, time-add, time-subtract.
* etc/NEWS: Mention the change.
* src/editfns.c (time_arith): Change last arg from function
to bool.  All callers changed.  Do the right thing with
infinities and NaNs.
(time_cmp): New function, which handlesx infinities and NaNs.
(Ftime_less_p): Use it.
(Ftime_equal): New function.
* test/lisp/emacs-lisp/timer-tests.el (timer-test-multiple-of-time):
Use it.