Add a sample "/etc/pam.conf" file that configures the authentication

Add a sample "/etc/pam.conf" file that configures the authentication
methods used by login.  Changes to "/usr/bin/login" to use it will
be committed later today.  The format of the file is described in
pam(8).

This sample file makes login behave in the traditional way.  To
wit, it enables authentication via S/Key and passwd/NIS lookups.
KerberosIV authentication is present in the sample file but commented
out.

As a safety net and a transition aid, login will fall back on
built-in passwd/NIS authentication if this configuration file is
missing or if some other fatal PAM error occurs.

This file will eventually replace "/etc/auth.conf", but not until
I've finished converting the other utilities, such as passwd and su.