Use a more backwards-compatible timer format.

Use a more backwards-compatible timer format.

* etc/NEWS: Document it.
* lisp/emacs-lisp/timer.el (timer): PSECS is now at the end, rather than
being right after USECS, as that better supports old code that
inadvisedly looked directly at the timer vector.
* src/keyboard.c (decode_timer): Get PSECS from the 8th (origin-0)
vector element, not from the 4th, since PSECS is now at the end.
(Fcurrent_idle_time): Doc fix.

Fixes: debbugs:12430