Improve the dialog(1) API in dialog.subr by adding f_dialog_default_store()
and f_dialog_default_fetch(). Operating similar to functions introduced by
SVN r251236 and r251242, these functions operate as a pair for helping track
the default-item data (for the --menu, --checklist, and --radiolist
widgets).

This replaces the direct usage of a global to store the data with an
abstract method for readability and to centralize the code.