Unbreak route6d.

Unbreak route6d.

Instead of breaking sendmsg(2) by adding unneeded space to its cmsg
item, add space to the cmsg used by recvmsg(2), where it will be used
to get the incoming packet hop limit.

Reported by several over the last years, and more recently by 'bsdsx',
who tested it against NetBSD route6d.  Also works against Quagga ripng.

ok deraadt@ sthen@