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

[Andriy Gapon]

Here's the latest version of the patches:
http://people.freebsd.org/~avg/zfsboot.patches.4.diff

John,
the first of the patches implements the approach that we previously discussed.
All arguments are passed starting at a fixed offset that should provide enough
space for extending argument list.  The first of the extended arguments should be
a size of the arguments (including the size field).  Then it's easy to write
something like:
struct xargs
{
	uint32_t size;
	...
};
...
struct xargs xargs;
xargs.size =  sizeof(xargs);
...
__exec(..., xargs);


Marius, Gavin,
patch 1f94d9a is my attempt of adapting your sparc64 ZFS code to my larger changes
in patch ae5a9c6.  I have the patches separate to facilitate the review.  They
should be committed together.  I have only compile-tested the sparc64/ofw part, so
it could have some grave bugs or omissions.
Could you please review and/or test this patch?
I will greatly appreciate any discussion, suggestions, help.

I again invite everyone else to take part in the review and testing.

-- 
Andriy Gapon