factory: tighten PAM configuration

Apparently PAM reacts differently on different systems (?) and if no
authoritative matching module is found might either succeed/fail,
depending on the system.

Let's lock this down explicitly, by hooking in pam_deny.so.

Of course, these PAM files are just examples, and no distro in its right
mind would ship these unmodified, but let's default to something safe.

Fixes: #12950