Hide struct ip6q, struct ip6asfrag, struct ip6_moptions,

Hide struct ip6q, struct ip6asfrag, struct ip6_moptions,
struct ip6po_rhinfo and struct ip6_pktopts behind _KERNEL.
The only bit userland may want from netinet6/ip6_var.h is
struct ip6stat.

The recent change to struct ip6po_rhinfo to use struct route
resulted in various build failures in ports because code
included netinet6/ip6_var.h without net/route.h.

OK tb@ sthen@