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.

PR: 277470
Reviewed by: philip, jhb, emaste
Differential Revision: https://reviews.freebsd.org/D48576

(cherry picked from commit c43ae7ab4bf89c2b274c1cbefe663c456e9211d1)