vfs: Implement larger streams in multiple xattrs

vfs: Implement larger streams in multiple xattrs

See the comment at the top of this patch for a description of the data
format.

The lowerlevel xattr_multi routines now take the raw stream name
instead of the xattr name because they have to build up the real xattr
name attached to the file themselves.

The patch is a bit larger than I would like it, but the mechanics in
the _multi() routines don't make sense in isolation, and the rest is
mostly handling the raw_stream_name instead of the xattr_name.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>