FR: Multilock primary target priority

Multilock treats all targets as equally valid, but there are cases where the player very much wants to hit what they've targeted and cares less about the rest. This could be achieved by allowing the missile to drop a lock to make room when the primary target is available, or by reserving one or more lock slots for the primary target even if it's not inside the lock cone. This could conceivably be a weapon table setting or a pilot-controlled targeting mode toggle.