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When doing a non-interactive installation, don't display an interactive

Description

When doing a non-interactive installation, don't display an interactive
warning about a filesystem which doesn't have a mountpoint. Presumably, the
person who wrote the install script knew what they were doing.

Submitted by: Brian Mueller <bmueller@panasas.com>
MFC after: 1 month
Sponsored by: Panasas
Differential Revision: https://reviews.freebsd.org/D12346