Third $ai path mode needed please

[Sessile-Nomad]

When using the alt1 $ai path mode ships are compelled to complete their paths, even when ordered to do something else, which can cause problems when fighters are scrambling from a hanger bay into combat (and possibly when landing during combat come to think of it.)

We really need to add an alt2 pathing mode that contains the enhancements from alt1 but allows path shortcutting. Pretty please with sugar on top :-)

[Goober5000]

@LuytenFS would like to take a shot at this