Lisp code shouldn't use set-time-zone-rule except through setenv.

Lisp code shouldn't use set-time-zone-rule except through setenv.

* time.el (display-time-world-list, display-time-world-display):
* time-stamp.el (time-stamp-string):
* vc/add-log.el (add-change-log-entry): Use setenv instead of
set-time-zone-rule.

* src/editfns.c (Fset_time_zone_rule): Document relationship with the
setenv function.

Fixes: debbugs:7337