pam.d: install hard link with same mode as target

Previously when using NO_ROOT we recorded a METALOG entry for the
pam.d/ftp hard link with a different file mode than the link target
pam.d/ftpd, which is not permitted.

This change is similar to 1dbb9994d4dd for .profile

Sponsored by: The FreeBSD Foundation

(cherry picked from commit f2865d7d550161bf4058425a6677a839c065bde4)