dwatch: install hard links with same mode as target

Previoulsy when using NO_ROOT we recorded METALOG entries for dwatch
hard links with different file modes than their link targets, which is
not permitted.

Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37477

(cherry picked from commit 7543a9c0280a0f4262489671936a6e03b9b2c563)