tzsetup(8): Refactoring to delete dialog(3) code

Little refactoring to complete the conversion to bsddialog(3),
not functional changes:

• Delete: remaining code for autosizing using BSDDIALOG_AUTOSIZE.
• Add: constants to delete magical numbers and to avoid unclear returned values.
• Delete: redundant 'conf.button.default_cancel=false', performed by bsddialog_initconf().
• Add: a new function message_zoneinfo_file() to merge features.

Approved by: bapt (mentor)
Differential Revision: https://reviews.freebsd.org/D34862