bsdinstall: Add repair scripts to installer menu

[leafoliage]

This patch adds a script, repair, with a new button to the installer menu, which helps users mount partitions and perform simple checks. This improves the installer by making repair routines more convenient when an installer is used to inspect and repair a system. It is a framework that can have more repairing functions added later on.

This patch is part of the code of the GSoC 2024 Project - Installer, whose work is committed in PR #1395 . This patch is created for the ease of getting this project's work commited.