stand/zfs: Fix const-qual warnings

The input buffer is read-only, update casts to match.

No functional change intended.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

Add SHA512, skein, large blocks support for loader zfs.

Updated sha512 from illumos.
Using skein from freebsd crypto tree.
Since loader itself is using 64MB memory for heap, updated zfsboot to
use same, and this also allows to support zfs large blocks.

Note, adding additional features does increate zfsboot code, therefore
this update does increase zfsboot code to 128k, also I have ported gptldr.S
update to zfsldr.S to support 64k+ code.

With this update, boot1.efi has almost reached the current limit of the size
set for it, so one of the future patches for boot1.efi will need to
increase the limit.

Currently known missing zfs features in boot loader are edonr and gzip support.

Reviewed by: delphij, imp
Approved by: imp (mentor)
Obtained from: sha256.c update and skein_zfs.c stub from illumos.
Differential Revision: https://reviews.freebsd.org/D7418

A new jail(8) with a configuration file, to replace the work currently done
by /etc/rc.d/jail.

Update ZFS from version 6 to 13 and bring some FreeBSD-specific changes.

This bring huge amount of changes, I'll enumerate only user-visible changes:

- Delegated Administration

Allows regular users to perform ZFS operations, like file system
creation, snapshot creation, etc.

- L2ARC

Level 2 cache for ZFS - allows to use additional disks for cache.
Huge performance improvements mostly for random read of mostly
static content.

- slog

Allow to use additional disks for ZFS Intent Log to speed up
operations like fsync(2).

- vfs.zfs.super_owner

Allows regular users to perform privileged operations on files stored
on ZFS file systems owned by him. Very careful with this one.

- chflags(2)

Not all the flags are supported. This still needs work.

- ZFSBoot

Support to boot off of ZFS pool. Not finished, AFAIK.

Submitted by: dfr

- Snapshot properties

- New failure modes

Before if write requested failed, system paniced. Now one
can select from one of three failure modes:
- panic - panic on write error
- wait - wait for disk to reappear
- continue - serve read requests if possible, block write requests

- Refquota, refreservation properties

Just quota and reservation properties, but don't count space consumed
by children file systems, clones and snapshots.

- Sparse volumes

ZVOLs that don't reserve space in the pool.

- External attributes

Compatible with extattr(2).

- NFSv4-ACLs

Not sure about the status, might not be complete yet.

Submitted by: trasz

- Creation-time properties

- Regression tests for zpool(8) command.

Obtained from: OpenSolaris