The function uses the internal service_user type, so it is not
really usable from the outside of glibc. Rename the function
to __nss_database_lookup2 for internal use, and change
__nss_database_lookup to always indicate failure to the caller.
__nss_next already was a compatibility symbol. The new
implementation always fails and no longer calls __nss_next2.
unscd, the alternative nscd implementation, does not use
__nss_database_lookup, so it is not affected by this change.
