Improve time stamp handling, and be more consistent about it.

This implements a suggestion made in:
http://lists.gnu.org/archive/html/emacs-devel/2014-10/msg00587.html
Among other things, this means timer.el no longer needs to
autoload the time-date module.
* doc/lispref/os.texi (Time of Day, Time Conversion, Time Parsing)
(Processor Run Time, Time Calculations):
Document the new behavior, plus be clearer about the old behavior.
(Idle Timers): Take advantage of new functionality.
* etc/NEWS: Document the changes.
* lisp/allout-widgets.el (allout-elapsed-time-seconds): Doc fix.
* lisp/arc-mode.el (archive-ar-summarize):
* lisp/calendar/time-date.el (seconds-to-time, days-to-time, time-since):
* lisp/emacs-lisp/timer.el (timer-relative-time, timer-event-handler)
(run-at-time, with-timeout-suspend, with-timeout-unsuspend):
* lisp/net/tramp.el (tramp-time-less-p, tramp-time-subtract):
* lisp/proced.el (proced-time-lessp):
* lisp/timezone.el (timezone-time-from-absolute):
* lisp/type-break.el (type-break-schedule, type-break-time-sum):
Simplify by using new functionality.
* lisp/calendar/cal-dst.el (calendar-next-time-zone-transition):
Do not return time values in obsolete and undocumented (HI . LO)
format; use (HI LO) instead.
* lisp/calendar/time-date.el (with-decoded-time-value):
Treat 'nil' as current time.  This is mostly for XEmacs.
(encode-time-value, with-decoded-time-value): Obsolete.
(time-add, time-subtract, time-less-p): Use no-op autoloads, for
XEmacs.  Define only if XEmacs, as they're now C builtins in Emacs.
* lisp/ldefs-boot.el: Update to match new time-date.el
* lisp/proced.el: Do not require time-date.
* src/editfns.c (invalid_time): New function.
Use it instead of 'error ("Invalid time specification")'.
(time_add, time_subtract, time_arith, Ftime_add, Ftime_less_p)
(decode_float_time, lisp_to_timespec, lisp_time_struct):
New functions.
(make_time_tail, make_time): Remove.  All uses changed to use
new functions or plain list4i.
(disassemble_lisp_time): Return effective length if successful.
Check that LOW is an integer, if it's combined with other components.
(decode_time_components): Decode into struct lisp_time, not
struct timespec, so that we can support a wide set of times
regardless of whether time_t is signed.  Decode plain numbers
as seconds since the Epoch, and nil as the current time.
(lisp_time_argument, lisp_seconds_argument, Ffloat_time):
Reimplement in terms of new functions.
(Fencode_time): Just use list2i.
(syms_of_editfns): Add time-add, time-subtract, time-less-p.
* src/keyboard.c (decode_timer): Don't allow the new formats (floating
point or nil) in timers.
* src/systime.h (LO_TIME_BITS): New constant.  Use it everywhere in
place of the magic number '16'.
(struct lisp_time): New type.
(decode_time_components): Use it.
(lisp_to_timespec): New decl.

lisp/emacs-lisp/timer.el

@@ -125,9 +125,7 @@ of SECS seconds since the epoch.  SECS may be a fraction."
   "Advance TIME by SECS seconds and optionally USECS microseconds
 and PSECS picoseconds.  SECS may be either an integer or a
 floating point number."
-  (let ((delta (if (floatp secs)
-		   (seconds-to-time secs)
-		 (list (floor secs 65536) (mod secs 65536)))))
+  (let ((delta secs))
     (if (or usecs psecs)
 	(setq delta (time-add delta (list 0 0 (or usecs 0) (or psecs 0)))))
     (time-add time delta)))
@@ -307,8 +305,8 @@ This function is called, by name, directly by the C code."
               ;; perhaps because Emacs was suspended for a long time,
               ;; limit how many times things get repeated.
               (if (and (numberp timer-max-repeats)
-                       (< 0 (timer-until timer (current-time))))
-                  (let ((repeats (/ (timer-until timer (current-time))
+                       (< 0 (timer-until timer nil)))
+                  (let ((repeats (/ (timer-until timer nil)
                                     (timer--repeat-delay timer))))
                     (if (> repeats timer-max-repeats)
                         (timer-inc-time timer (* (timer--repeat-delay timer)
@@ -374,13 +372,13 @@ This function returns a timer object which you can use in `cancel-timer'."
 
   ;; Handle numbers as relative times in seconds.
   (if (numberp time)
-      (setq time (timer-relative-time (current-time) time)))
+      (setq time (timer-relative-time nil time)))
 
   ;; Handle relative times like "2 hours 35 minutes"
   (if (stringp time)
       (let ((secs (timer-duration time)))
 	(if secs
-	    (setq time (timer-relative-time (current-time) secs)))))
+	    (setq time (timer-relative-time nil secs)))))
 
   ;; Handle "11:23pm" and the like.  Interpret it as meaning today
   ;; which admittedly is rather stupid if we have passed that time
@@ -486,7 +484,7 @@ The value is a list that the debugger can pass to `with-timeout-unsuspend'
 when it exits, to make these timers start counting again."
   (mapcar (lambda (timer)
 	    (cancel-timer timer)
-	    (list timer (time-subtract (timer--time timer) (current-time))))
+	    (list timer (time-subtract (timer--time timer) nil)))
 	  with-timeout-timers))
 
 (defun with-timeout-unsuspend (timer-spec-list)
@@ -495,7 +493,7 @@ The argument should be a value previously returned by `with-timeout-suspend'."
   (dolist (elt timer-spec-list)
     (let ((timer (car elt))
 	  (delay (cadr elt)))
-      (timer-set-time timer (time-add (current-time) delay))
+      (timer-set-time timer (time-add nil delay))
       (timer-activate timer))))
 
 (defun y-or-n-p-with-timeout (prompt seconds default-value)