bsdinstall hostname: Replace dialog with bsddialog

Replace (LGPL) dialog utility with (BSD-2-Clause) bsddialog utility.
Add --hline info because bsddialog requires TAB to switch the focus on
buttons for an inputbox, dialog differs.
Delete f_dialog_inputbox_size() using libbsddialog autosizing.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D35285