r3545: initial support for using extended attributes to hold extended dos attributes...

r3545: initial support for using extended attributes to hold extended dos attributes of files.

I decided to use IDL/NDR to encode the attribute, as it gives us a
simple way to describe and extend the saved attributes.

The xattr code needs to hook into quite a few more places in the pvfs
code, but this at least gets the basics done. I will start encoding
alternate data streams streams, DOS EAs etc soon using the same basic
mechanism.

I'll probably stick to "version 1" for the xattr.idl for quite a while
even though it will be changing, as I don't expect anyone to be
deploying this in production just yet. Once we have production users
we will need to keep compatibility by supporting all the old version
numbers in xattr.idl.
(This used to be commit c54253ed1b7dce1d14f43e747da61089aea87094)