#510: Fix disappearing AIShips.

#510: Fix disappearing AIShips.
Untangle AI reinit and init methods. Some code in init hooks expects to be
called once only. Derived classes should not redirect their reinit to init,
even if this seems fine for the methods of the derived class itself. This
also triggers the init methods of all base classes, which may not expect
multiple calls to their init methods (or to "init" on "reinit").