bsddialog: Fix for terminals without colours

When running the installer, in particular disextract (which is so far
the only part converted to bsddialog), on serial console or vt100 or
actually any terminal without color support, it failed to start.

This change makes bsddialog fallback on the black and white theme.

This is incorporated in newer version of bsddialog which will be
imported soon.

PR: 261272
Reported by: thj
Differential Revision: https://reviews.freebsd.org/D33920