libsmack: use 16 bits for smack access codes instead of sizeof(int) * 2

There are 6 access bits to be stored in the access field. Special value -1
is used to distinguish set rules from change rules. Shrinking the filed to
8 bits still leaves space for one more future access bit.

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>