bsdinstall/partedit: Fix UFS auto partitioning

Fix bsdinstall "Auto (UFS) Guided Disk Setup" and sade(8) "Auto".
The problem is a string comparison failure, it arose during the
dialog(3)/bsddialog(3) form conversion:

• dialog uses only form.text while bsdialog differentiates between form.init and form.value.
• dialog always allocates memory for form values while bsddialog only when a button is pressed.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D35033