bsdinstall hostname: Restore bsdconfig API

Restore bsdconfig API to handle hostname dialog because bsddialog 1.0
matches all features required by bsdconfig(1). Implicitly this commit
restores also Xdialog(1) for this script.

This commit reverts:

• "bsdinstall hostname: Replace dialog with bsddialog" 6368dcb29228dd8e18d50c54c3ca1596262d4676
• "bsdinstall(8) hostname: Update for bsddialog 0.3" 5f3ec44e7e9c11f90ad1128db1116925b493fad0.

The script continues to use bsddialog(1) by default via $DIALOG in
bsdconfig(1).