Only retrieve password policies in pam_auth when WBFLAG_PAM_GET_PWD is set.

Only retrieve password policies in pam_auth when WBFLAG_PAM_GET_PWD is set.

This essentially re-establishes r14496 (2155bb0535656f294bd054d6a0a7d16a9a71c31b)
which was undone in r17723 (43bd8c00abb38eb23a1497a255d194fb1bbffffb) for
reasons that are unclear to me. Maybe I am being too naive.

Now we do again only retrieve the password policy when called from
the pam_winbind module. This fixes logons delegated to AD trusted
domain controllers: We need to connect to the sam to retrieve the
password policy. But auhtenticated session setup is not possible
when contacting the trusted domain dc and afterwards, SamrConnect
also fails with whatever credentials and method used.

Michael