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
94cb21cda6