sss_client: Work around glibc bug
glibc is inconsistent with how it treats and returns NSS_STATUS_UNAVAIL.
The sss nss plugin is present in nsswitch by default on some platforms
due to glibc caching and problem with long living applications (e.g. GNOME).
But sssd needn't be configuread and it cause problems in some programs.
In this situation, the SSSD nss plugin should behave as if it was functioning
but had no data even thought sssd is not running. The errors have to be passed
from nss plugin up to the user with minimal moidiffication.
Thanks to Stephen Gallagher for initial patch.
Resolves:
https://fedorahosted.org/sssd/ticket/2439
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>