apparmor: group dfa policydb unpacking

There are currently three policydb rule groupings (xmatch, file,
policydb) that each do their own slightly different thing. Group them
into a single routine and unify.

This extends/unifies dfa features by
- all dfas are allowed having an optional start field
- all dfas are allowed having a string/transition table

Signed-off-by: John Johansen <john.johansen@canonical.com>