nss_winbind: When returning NSS_UNAVAIL, squash errno to ENOENT
According to the GNU libc nss guide, we should always set
errno to ENOENT when returning NSS_UNAVAIL.
http://www.gnu.org/software/libtool/manual/libc/NSS-Modules-Interface.html#NSS-Modules-Interface
At least the MQ Series message queing service that runs
on WebSphere will fail if you return any other errno in this case.
(cherry picked from commit
ee26664602445fa7798e2061f6bcbef0756d6528)
(cherry picked from commit
29b39723b82f363d32dc4678d6b71a78485c65ce)