- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

zfs boot: export boot/primary pool and vdev guid all the way to kenv

This is work in progress to for znextboot and it also provides
some convenient infrastructure.

MFC after: 20 days

zfs boot: add code for listing child datasets of a given dataset

- only filesystem datasets are supported
- children names are printed to stdout

To do: allow to iterate over the list and fetch names programatically

MFC after: 17 days

zfsboot/zfsloader: support accessing filesystems within a pool

In zfs loader zfs device name format now is "zfs:pool/fs",
fully qualified file path is "zfs:pool/fs:/path/to/file"
loader allows accessing files from various pools and filesystems as well
as changing currdev to a different pool/filesystem.

zfsboot accepts kernel/loader name in a format pool:fs:path/to/file or,
as before, pool:path/to/file; in the latter case a default filesystem
is used (pool root or bootfs). zfsboot passes guids of the selected
pool and dataset to zfsloader to be used as its defaults.

zfs support should be architecture independent and is provided
in a separate library, but architectures wishing to use this zfs support
still have to provide some glue code and their devdesc should be
compatible with zfs_devdesc.
arch_zfs_probe method is used to discover all disk devices that may
be part of ZFS pool(s).

libi386 unconditionally includes zfs support, but some zfs-specific
functions are stubbed out as weak symbols. The strong definitions
are provided in libzfsboot.
This change mean that the size of i386_devspec becomes larger
to match zfs_devspec.

Backward-compatibility shims are provided for recently added sparc64
zfs boot support. Currently that architecture still works the old
way and does not support the new features.

TODO:
- clear up pool root filesystem vs pool bootfs filesystem distinction
- update sparc64 support
- set vfs.root.mountfrom based on currdev (for zfs)

Mid-future TODO:
- loader sub-menu for selecting alternative boot environment

Distant future TODO:
- support accessing snapshots, using a snapshot as readonly root

Reviewed by: marius (sparc64),
Gavin Mu <gavin.mu@gmail.com> (sparc64)
Tested by: Florian Wagner <florian@wagner-flo.net> (x86),
marius (sparc64)
No objections: fs@, hackers@
MFC after: 1 month