allow npc ship respawning to be optional

lua script scenarios might not want ships to auto-respawn randomly.
fix min/max attack vector distance code

Distance should take object radius into account
add ai_push_attack function to lua api
npc ships should not give up attacking stationary objects that are too far away

There is logic to give up attacking ships that are "too far" away.
This logic should not apply for objects that are stationary.
adjust firing range to account for planet radii
fix planet-torpedo collision detection bug

It was mistakenly impossible for torpedoes to collide with planets
Fix npc ship firing range calculations to take into account large planet radii
and do not prevent npc ships firing at planets "because there is a planet in
the way".