if: Add ioctls to get RSS key and hash type/function.

It will be needed by hn(4) to configure its RSS key and hash
type/function in the transparent VF mode in order to match VF's
RSS settings. The description of the transparent VF mode and
the RSS hash value issue are here:
https://svnweb.freebsd.org/base?view=revision&revision=322299
https://svnweb.freebsd.org/base?view=revision&revision=322485

These are generic enough to promise two independent IOCs instead
of abusing SIOCGDRVSPEC.

Setting RSS key and hash type/function is a different story,
which probably requires more discussion.

Comment about UDP_{IPV4,IPV6,IPV6_EX} were only in the patch
in the review request; these hash types are standardized now.

Reviewed by:    gallatin
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D12174