Improvements for autonomous missiles

[CopyOfSatan]

@AuroraSteve

Missile Self-Targeting

In v2.6, when a missile has onboard sensors but no current target, it will use those active, thermal or EM sensors (in that order) to search for a ship target. The closest valid target is selected.

For v2.7, the target selection will be a random valid target within sensor range, weighted by chance to hit.

The choice of target is determined by the chance of a missile hit.