nscd: allow using systemd-userdb
If systemd-userdb is used to configure some users, nscd needs to read the userdb files.
See also https://www.freedesktop.org/software/systemd/man/nss-systemd.html for the list of possible filenames (and symlinks).
Fixes: http://bugzilla.opensuse.org/show_bug.cgi?id=1207698
I propose this patch for all branches.