This allows restricting each CA individually to only a subset of the units. It also means that several instances of the same CA can be used in the same AI configuration, in order to, for example, force the order in which units are dealt with, or to apply different settings to different units.
previously the unit ctor could invoke arbitrary umc lua code, for
example via advance_to -> add_modification ->
resources::lua_kernel->apply_effect and during that call the unit was
not owned by a unit_ptr, so the code could not use unit_ptr objects at
several places because the unit might not be owned by a unit_ptr object.
This constitutes drop-in replacements for:
* boost::shared_ptr
* boost::scoped_ptr
* boost::weak_ptr
* boost::enable_shared_from_this
* boost::static_pointer_cast
* boost::dynamic_pointer_cast
This excludes boost::intrusive_ptr, except for stray includes. Refactoring that is more complicated.
