[review request] zfsboot/zfsloader: support accessing filesystems within a pool

[Andriy Gapon]

I would like to ask for a review and/or testing of the following three patches:
http://people.freebsd.org/~avg/zfsboot.patches.diff

These patches add support for booting from an arbitrary filesystem of any
detected ZFS pool.  A filesystem could be selected in zfsboot and thus will
affectfrom where zfsloader would be loaded.  zfsboot passes information about
the boot pool and filesystem to zfsloader, which uses those for loaddev and
default value of currdev.  A different pool+filesystem could be selected in
zfsloader for booting kernel.  Also if vfs.root.mountfrom is not explicitly set
and is not derived from fstab, then it gets set to the selected boot filesystem.

This should could be used as a foundation for the support of Solaris-like boot
environment selection.  I believe that other people have already developed
scripts utilizing ZFS capabilities to provide other aspects of management of
boot environments.

I am particularly interested in reviews of my attempt to make ZFS boot support
arch-independent.  The arches, of course, would have to add some code to make
use of that support.  Currently I only enabled it for x86.

Thank you very much!
-- 
Andriy Gapon