dma: Don't install dma.conf world-readable

While here, install auth.conf into /etc as well.

MFC after: 3 days
PR: 288409
Reviewed by: ivy, bapt
Differential Revision: https://reviews.freebsd.org/D51529

(cherry picked from commit a3d4ae7cf351cc283a670d8400a89518a6b31d63)