this changes function prototype for __ivalid*.
This commit breaks binary compatibility for __ivalid*. I believe this can be
forgiven due to the following:
- this is not really exported function. no function prototype is in headers.
function name starts with underbars. No third-party applications are
expected to use it.
- the function was introduced very recently, when rcmd.c was made IPv6-ready.
- the only customer in NetBSD tree is lpd.
(lpd.c) sync with rcmd.c change.