nss: Turn __nss_database_lookup into a compatibility symbol

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.