nss: Use macro to generate public API names

nss: Use macro to generate public API names

The name of the exported functions for an NSS module is quite
fixed, it is derived from the module name:

  _nss_$module_$function

Since we will create another NSS module with very similar
implementation we might as well generate the function names at
the compile time.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>