This allows you to save the errno from a previous call and
pass it to reply_with_perror.
For example, original code:
r = some_system_call ();
err = errno;
do_cleanup ();
errno = err;
if (r == -1) {
reply_with_perror ("failed");
return -1;
}
can in future be changed to:
r = some_system_call ();
err = errno;
do_cleanup ();
if (r == -1) {
reply_with_perror_errno (err, "failed");
return -1;
}