libsmack: change logic for detecting long labels in smack_accesses

Instead of storing label length in each smack_rule, have one integer in
smack_accesses to remember if long labels are used.
This saves few bytes per rule.

Signed-off-by: Rafal Krypa <r.krypa@samsung.com>
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.
Fields in smack_rule have been reordered for optimal alignment.

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