labels of 255 characters can't be set with chsmack

Setting labels of 255 characters causes chsmack to invoque lsetxattr for a value of 256 bytes what is rejected as "invalid argument".

It seems that smack includes a terminating null char to it's setting for label.

A decision has to be taken (it is maybe already taken, i don't know...):
 - tell that smack labels are strings up to 254 characters
or
 - review the code to ensure that the terminating null for strings of 255 characters can be removed