Change current-time back to list form

Change current-time back to list form

Change current-time and related functions back to using the
traditional list form.  Also, add a new boolean variable
current-time-list that lets people try out (TICKS . HZ) form,
with the goal of smoothing the transition.
* src/timefns.c (CURRENT_TIME_LIST): Change default back to true.
(current-time-list): New boolean Lisp variable, which defaults to
CURRENT_TIME_LIST.  All uses of CURRENT_TIME_LIST changed to
use current_time_list, and all documentation changed.