Drop HAVE_XVATTR macros

Drop HAVE_XVATTR macros

When I began work on the Posix layer it immediately became clear to
me that to integrate cleanly with the Linux VFS certain Solaris
specific things would have to go.  One of these things was to elimate
as many Solaris specific types from the ZPL layer as possible.  They
would be replaced with their Linux equivalents.  This would not only
be good for performance, but for the general readability and health of
the code.  The Solaris and Linux VFS are different beasts and should
be treated as such.  Most of the code remains common for constructing
transactions and such, but there are subtle and important differenced
which need to be repsected.

This policy went quite for for certain types such as the vnode_t,
and it initially seemed to be working out well for the vattr_t.  There
was a relatively small amount of related xvattr_t code I was forced to
comment out with HAVE_XVATTR.  But it didn't look that hard to come
back soon and replace it all with a native Linux type.

However, after going doing this path with xvattr some distance it
clear that this code was woven in the ZPL more deeply than I thought.
In particular its hooks went very deep in to the ZPL replay code
and replacing it would not be as easy as I originally thought.

Rather than continue persuing replacing and removing this code I've
taken a step back and reevaluted things.  This commit reverts many of
my previous commits which removed xvattr related code.  It restores
much of the code to its original upstream state and now relies on
improved xvattr_t support in the zfs package itself.

The result of this is that much of the code which I had commented
out, which accidentally broke things like replay, is now back in
place and working.  However, there may be a small performance
impact for getattr/setattr operations because they now require
a translation from native Linux to Solaris types.  For now that's
a price I'm willing to pay.  Once everything is completely functional
we can revisting the issue of removing the vattr_t/xvattr_t types.

Closes #111