Per-turret disable/enable/burst mode

Similar to individual engine selection, except for turrets.

Each turret can be enabled, disabled or in burst mode. Burst mode isn't offered for turrets with no burst capability. When selecting turrets, screen will center on highlighted turret's location with an 'X' denoting its position.

Adds a new int to amount/direction/open union (turrets didn't use this union before, right?). Currently uses only values 0, 1 and 2 for disabled, enabled and burst.

By default all newly installed turrets are disabled.
This is very annoying: I installed a flamethrower and activated turrets, for specific turrets the default mode seems to be 0, so the new turret was still inactive (mode==0) and this function returned 0, which imideately shut down all turrets, see `vehicle::gain_moves` line 4330 with this message: "The %s's turrets run out of ammo and switch off."