roverlay/overlay/abccontrol: get effective PKG_ mask

add get_effective_package_policy()
package rules, addition control: change "default"

... action: forcefully set overlay_addition_override to DEFAULT
addition control rule gen: partial implementation

Refinement and implementation/refinement of steps 2..5.

Todo list for the addition control generator module:
* add category, package token objects (e.g. tuples)
* step 1: read+create tokens and create the acceptor_chain -> bitmask map
* step 5: implement a package rule generator object with proper
           {category,package}_token_to_acceptor() methods

* remove SillyAdditionControlPackageRuleGenerator,
   which translates all tokens into TrueAcceptors
   (not useful anyway, just for testing while a real gen. is not implemented)