o Make net.inet6.ip6.mif6table return special API structure, that doesn't

contain kernel pointers, and instead has interface index.
Bump __FreeBSD_version for that change.

o Now, netstat/mroute6.c no longer needs to kvm_read(3) struct ifnet, and

no longer needs to include if_var.h

Note that this change is far from being a complete move of IPv6 multicast
routing to a proper API. Other structures are still dumped into their
sysctls as is, requiring userland application to #define _KERNEL when
including ip6_mroute.h and then call kvm_read(3) to gather all bits and
pieces. But fixing this is out of scope of the opaque ifnet project.

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