WIP: Multiple policy cache directories

Implement support for multiple policy cache directories. A new policy cache dir will exist for each unique set of AppArmor kernel features that the parser sees. By default, the policy cache directories are located in /etc/apparmor.d/cache.d/. Each policy cache directory is a 8 character string which is the output of a djb2 hash where the input is the features string that's constructed from apparmorfs (or a different source if specified on the parser command line).

This results in cache directories such as /etc/apparmor.d/cache.d/00001505/ and /etc/aparmor.d/cache.d/db76596e/. Their contents are identical to the contents of /etc/apparmor.d/cache/ today.

Admins can discover the location of the policy cache directory path of the currently running kernel by passing the --print-cache-dir option to apparmor_parser.

Outside of general discussion about this approach, there are some known issues with this merge request that need addressing before the "WIP" prefix can be removed from the merge request:

• The parser does not yet expose the max_caches parameter of aa_policy_cache_new() to admins.
• Hash collisions are not detected/handled. (Is this needed?)
• Tests are needed to identify changes in the <features_id> calculations so that we don't unknowingly modify the algorithm.
• Tests are needed to stress this feature some more.
• Consider how this feature melds with policy versioning.