* add subsystem indexer and ensure that ship_get_indexed_subsys and ship_get_first_subsys are independent
* also add reverse lookup from subsys to index
* update indexer to accommodate subsystem pointers being changed
* use clearer function names, and fix the type of subsys_list_indexer
* add assertions
* address latest feedbcak
This adds a new AI goal which is equivalent to play-dead except that it "sticks". It doesn't clear the goal list
so it will continue playing dead until the goal is removed or another goal with a higher priority is added.
Goober5000