Smack: implement setselfattr and getselfattr hooks

Implement Smack support for security_[gs]etselfattr.
Refactor the setprocattr hook to avoid code duplication.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>