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 avoiding unclear return values.
• Delete: redundant 'conf.button.defaultno=false', performed by bsddialog_initconf()
• Add: a new function message_zoneinfo_file() to merge features.

Not functional changes, just some constant and a trivial function.