The ai-fly-to-ship goal was added early in FSO's development but never worked right. Until recently it could crash; and even aside from the crash the AI would try to reach the target's exact position, causing repeated collisions. This folds ai-fly-to-ship into ai-stay-near-ship, since the behaviors are conceptually the same except for fly-to being satisfied as soon as the target is in range.
This also adds a new "escort" mode usable by both behaviors. The default Volition behavior is to swoop toward the target in a long, graceful arc; the new mode generates a goal point and flies directly to that point. These can be toggled via an optional sexp parameter.
There is also some minor cleanup and a new vec3d operator for convenience.
Needed for Scroll; depends on #5667.
Goober5000
b89cbbd530