smack: add builtin Smack rule comparisons

The Smack LSM has a series of hardcoded rules that it consults before
the rules loaded and cached in buxton's hashmap.

This commit adds support for these hardcoded rules, implementing checks
in the same order as Smack.
Define BuxtonSmackLabel type and use it

Instead of passing char* around, it makes more sense to add a type for
the Smack labels handled internally by buxton.
Add tests for builtin Smack rule checks
Minor cleanup for the Smack access tests
Add one more Smack access test for loaded rules