[CFR] ZFS features support

[Martin Matuska]

I have now completed boot support for ZFS features.

Please review and comment my patch:
http://people.freebsd.org/~mm/patches/zfs/features/head-zfs-features.patch

Boot-only patch:
http://people.freebsd.org/~mm/patches/zfs/features/head-zfs-features-boot.patch

I have also an alternate boot patch that adds functions like
nvlist_name(), nvlist_value(), but the previous one has a smaller footprint:
http://people.freebsd.org/~mm/patches/zfs/features/head-zfs-features-boot.fc.patch

The boot code can be tested with the "zhack" tool, e.g. with:
zhack feature add tank org.freebsd:test
zhack feature ref -m tank org.freebsd:test
Now you are unable to boot.
zhack feature ref -d tank org.freebsd:test
Now you are able to boot.

If there are no quick objections, I will commit this to -HEAD with a
1-month MFC.

Cheers,
mm

-- 
Martin Matuska
FreeBSD committer
http://blog.vx.sk