[SELINUX]: add security class for appletalk sockets

Add a security class for appletalk sockets so that they can be
distinguished in SELinux policy.  Please apply.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>