PAM: new options pam_trusted_users & pam_public_domains
pam_public_domains option is a list of numerical UIDs or user names
that are trusted.
pam_public_domains option is a list of domains accessible even for
untrusted users.
Based on:
https://fedorahosted.org/sssd/wiki/DesignDocs/RestrictDomainsInPAM
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>