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>

tests/nsslinktest.c

@@ -33,7 +33,7 @@ int main(int argc ATTRIBUTE_UNUSED,
      * the fact this test has been built successfully means
      * there's no linkage problem and therefore success is
      * returned. */
-    _nss_libvirt_gethostbyname_r(NULL, NULL, NULL, 0, &err, &herrno);
+    NSS_NAME(gethostbyname)(NULL, NULL, NULL, 0, &err, &herrno);
 
     return EXIT_SUCCESS;
 }