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/pwmem.c

@@ -48,6 +48,8 @@
 	if (NULL == pw) {
 		return NULL;
 	}
+	/* The libc might define other fields. They won't be copied. */
+	memset (pw, 0, sizeof *pw);
 	pw->pw_uid = pwent->pw_uid;
 	pw->pw_gid = pwent->pw_gid;
 	pw->pw_name = strdup (pwent->pw_name);