Stops idle syndicate mobs from destroying surroundings

Adds a variable ('idle_env_destroyer') to all hostile mobs, denoting
whether the mob destroys it's environment while idling or not. Default
value is 0 (no). Syndicate, russian and pirate mobs are affected. Also
prevents aforementioned mobs from suiciding and/or wrecking away mission
environments before the player could see them.

Fixes hostile mobs interpreting 'null' as a valid target and always
being in HOSTILE_ATTACK stance.