etcupdate: Restrict access to the conflicts directory

In the window during conflict resolution, copies of installed files with
conflicts are added here with the default mode. Restrict access.

Approved by: so
Security: FreeBSD-SA-25:03.etcupdate
PR: 277470
Reviewed by: philip, jhb, emaste
Differential Revision: https://reviews.freebsd.org/D48576

(cherry picked from commit c43ae7ab4bf89c2b274c1cbefe663c456e9211d1)
(cherry picked from commit 17e935f1f327d7d4464e53f4f3d2347a51623f82)