Add a BuxtonKeyAccessType enum

This enum represents the access types associated with each Smack rule
enforced by the kernel.  Smack supports more access types than 'read'
and 'write', but these are the ones that are useful for buxton.
Add a function for caching Smack rules

For fast lookups of Smack rules and their access permissions, store the
entire rule set in a hashmap.

The format of the hashmap keys is "subject object", and the values are
of type BuxtonKeyAccessType, corresponding to "r", "w", or "rw" access
permissions.
Add function for checking Smack access

After the Smack labels from client (subject) and the target key (object)
have been identified, this function can be used for checking whether the
requested Smack access (read or write) for the subject-object pair is
permitted.