tzsetup: symlink /etc/localtime instead of copying

Using a symlink means that new timezone data (installed by an errata
update, say) will be usable without having to be copied again.

Reviewed by: bapt, kevans, philip
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37005

(cherry picked from commit 5e16809c953f4cd19fadb1767469dec319de0353)