Make locked-homing weapons lose their target when it dies

[Goober5000]

When a locked-homing (aspect/javelin) weapon's target was destroyed or departed, its homing_object kept pointing at the freed object slot, so "die on lost lock" never fired. If the slot was reused by an unrelated weapon, the missile would chase it, flying in circles until its lifetime expired.

weapon_home() now checks the target signature once for locked-homing weapons. On a mismatch, find_homing_object_by_sig() re-acquires the original target or, if it is gone, clears homing_object, homing_subsys, target_num, and target_sig. A countermeasure that decoyed the weapon remains the only allowed mismatch, and the weapon still re-acquires its original target when the countermeasure expires. A countermeasure the weapon can't home on still breaks its lock, and now clears the same fields and sends a multiplayer homing update.

This changes behavior for all locked-homing weapons, not just those with "die on lost lock". Once their target is destroyed or departs, missiles without the flag now fly straight instead of chasing whatever object reused the target's slot, and missiles with the flag now self-destruct, including when the target warps out. Since a missile whose target is gone could only score incidental hits either way, this doesn't affect balance, so it isn't gated behind an AI profile flag.

Also, a locked-homing weapon fired without a lock that is decoyed by a countermeasure it can't home on now ignores the countermeasure and keeps flying straight, as a locked weapon already did. Previously it would turn and chase the countermeasure.

Fixes #7849.