NSS: Replace exported NSS lookup functions with stubs [BZ #21962]

Commit 384ca55174 from 2007 added this to
nss/XXX-lookup.c:

+#ifndef NO_COMPAT
+int
+internal_function attribute_compat_text_section
+DB_COMPAT_FCT (service_user **ni, const char *fct_name, void **fctp)
+{
+  return DB_LOOKUP_FCT (ni, fct_name, NULL, fctp);
+}
+#endif

That is, it adds a pseudo-compat function with an internal_function
attribute.  The function it was supposed to replace did not have the
attribute:

 extern int DB_LOOKUP_FCT (service_user **ni, const char *fct_name,
-			  void **fctp) internal_function;
+			  const char *fct2_name, void **fctp)
+  internal_function;

This changed the calling convention on i386 for the following
functions in the public ABI:

  __nss_passwd_lookup
  __nss_group_lookup
  __nss_hosts_lookup

This commit replaces the functions with always-failing stubs,
with true compat symbols.  Due to a happy accident, the calling
convention of the stub is identical for the internal_function
and non-internal_function case on i386.

In addition, this commit auto-generates the __nss_*_lookup2
function declarations as part of <nsswitch.h>.

grp/initgroups.c

@@ -36,11 +36,6 @@ typedef enum nss_status (*initgroups_dyn_function) (const char *, gid_t,
 						    long int *, long int *,
 						    gid_t **, long int, int *);
 
-/* The lookup function for the first entry of this service.  */
-extern int __nss_group_lookup (service_user **nip, const char *name,
-				   void **fctp);
-extern void *__nss_lookup_function (service_user *ni, const char *fct_name);
-
 extern service_user *__nss_group_database attribute_hidden;
 service_user *__nss_initgroups_database;
 static bool use_initgroups_entry;