Update after emaste, markj and bcr review.

Oh sorry @emaste I some how didn't get the emails from your comments. I will address them ASAP.

In D24920#549047, @tsoome wrote:

In general, it seems nice. I'd like to see more for description; how the checkpoints would appear, what it means to the boot process, would it mean updates for some manual/handbook? It would nice to refer to zpool, not all people do know where to look for information.

Typo pointed by @kevans. Thanks!

Thank you @bcr and @greg_unrelenting.technology

Man pages fixes.

If you want I can commit this for you.

Some more notes.

Oh sorry I miss read the patch.
Then this looks good to me :)

This doesn't break multiple encrypted disks?
If I have 3 disk and only one should be decrypted by the loader?

Like Matt suggested I checked the behavior on Linux as well.

Thank you for working on this!
This doesn't seems like simple peace of code.

LGTM.
Like @markj mentioned please add it to man page.

Ping.

Fix value usage.

@kib I introduced a helper function.

Update diff.

Yes. I mess it up while refactoring it.

Address @markj comments.

Thank you @pjd.

I'm fine with naming nop devices any name as long as there is a nop suffix.
Otherwise somebody may do some weird stuff in /dev/.

Thx bcr@ !

Update description

2. I'm not sure. If the process used explicit pid in wait shouldn't we allow to trace it?