Little refactoring to complete the conversion to bsddialog(3), not functional changes:
* Delete remaining code for autosize using BSDDIALOG_AUTOSIZE.
* Add constants to delete magical numbers and avoiding unclear return values.
* Delete redundant 'conf.button.defaultno=false', performed by bsddialog_initconf()
* Add a new function message_zoneinfo_file() to merge features.
(cut ~50 lines).