More aircraft/shroud interaction fixes

This is a continuation of #5967, which only made planes not lose their selection when they strayed
outside the map border. This PR makes it possible to select them when they already are outside the map.

Fixes #5651 for real, then.

Also fixes #6001 by making aircraft not attack when they are outside the map. The change will affect both Yaks and Migs, even though Yaks did not have the original problem.
No.  We don't want the selection box to be drawn when the actor is an enemy AND beneath the shroud.

```||``` would mean that you wouldn't ever see the selection box for enemies, or for any units beneath the shroud.