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
a46d7ffeef1807bafda15eb76ec74fcf41aae7f6)