Fix Debian bug #675824

* lib/groupmem.c (__gr_dup): Support libc which define other
	fields in struct group.
	* lib/pwmem.c: Likewise for struct passwd.
	* lib/shadowmem.c: Likewise for struct spwd.
	* lib/sgroupio.c: Apply same logic, even if this structure is
	defined internally.

lib/sgroupio.c

@@ -51,6 +51,9 @@
 	if (NULL == sg) {
 		return NULL;
 	}
+	/* Do the same as the other _dup function, even if we know the
+	 * structure. */
+	memset (sg, 0, sizeof *sg);
 	sg->sg_name = strdup (sgent->sg_name);
 	if (NULL == sg->sg_name) {
 		free (sg);