Fix build of tzsetup when WITHOUT_DIALOG is set

Hide dialog specific code behind HAVE_DIALOG. It allows to build a stripped
down version (missing the dialog UI) but perfectly function tzsetup when
world is built WITHOUT_DIALOG

Reorganise a bit the code to limit the number of blocks under HAVE_DIALOG

Reviewed by: emaste
Sponsored by: https://reviews.freebsd.org/D8325