librpc/idl: rename NFS4 ACL xattr name

librpc/idl: rename NFS4 ACL xattr name

The "system" xattr namespace is reserved for the kernel. Any attempt to
use xattrs in that namesspace will fail with EOPNOTSUPP, regardless of
priveleges. In autobuild we're using the xattr_tdb VFS module, so it
works there.

Using the "security" namespace instead makes this module generally
usable with Linux filesystem xattrs as storage backend.

Additionally prefix the xattr name with "_ndr". This is in preperation
of later commits that add a ACL blob marshalling format based on XDR. To
avoid xattr name collision, both format will use distinct xattr names by
default.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>