bsdinstall/partedit: Replace libdialog with libbsddialog

bsdinstall/partedit: Replace (LGPL) libdialog with (BSD-2-Clause)
libbsddialog. Rewrite diskeditor.c and rename diskmenu.c because
it uses an API for menu totally incompatible with libbsddialog.
This is a User Interface change everything else is unchanged.

Approved by: bapt (mentor)
Differential Revision: https://reviews.freebsd.org/D34639