tzsetup: make UTC the first (default) region

Many sysadmins prefer to configure their systems to UTC and it's a
reasonable default when installing, making it easier to get a usable
system by just hitting <return> repeatidly.

Renumber UTC to 0 to preserve the finger memory of those selecting a
region by shortcut.

Reviewed by: jrtc27, emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42383