WIP: Add CR_MISSILE carry type to fix jumping off of old Brak Eggman missiles

These missiles were using CR_GENERIC as of 2.2 - the default behavior for CR_GENERIC removes the tracer's target when the player jumps off, causing the player to immediately latch back onto the missile. CR_MISSILE is introduced to rectify this issue - the controllable missile code is tied into this as well, so modders will be able to use this for hang gliders and such instead of relying on hardcoded behavior tied to the MT_BLACKEGGMAN_MISSILE object type.