tzsetup: be explicit about *timezone* abbreviation

During the install process tzsetup asks a question like

Does the abbreviation `EDT' look reasonable?

The installer asks lots of questions, some that relate to the previous
screen or topic and some that do not. A new user installed FreeBSD for
the first time and was confused by this question, not realizing that it
was asking whether the abbreviation is correct for the selected
timezone.

Reviewed by: bapt, brooks, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44500