Add capturable technology support.

This adds support for building units from other factions when you capture their production structures.  This adds plumbing for both shared and per-actor production queues, but only implements the yaml rules for TD.  a82764aa3cb9cea5945ea7385148b80f663271e8 shows how this can be done for the other mods.

The race prerequsites can also be used for faction-specific support powers on shared structures.
You probably forgot `= null` here.