Fix customization of zoneinfo-style-world-list

Fix customization of zoneinfo-style-world-list

A customizable variable's initial value cannot depend on that of
another customizable variable, since the variables are initialized
in other than textual order.  Problem reported by N. Jackson
(Bug#24291).
* lisp/time.el (display-time-world-list): Default to t,
a special value that expands to zoneinfo-style-word-list
if that works, and to legacy-style-word-list otherwise.
(time--display-world-list): New function.
(display-time-world, display-time-world-timer): Use it.