nss_winbind: When returning NSS_UNAVAIL, squash errno to ENOENT

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.