Fix carryall not removing influence when cargo dies

[PunkPun]

Fixes #20468
An oversight from #16509 and then #20403

Here we make sure we properly handle cancelling regardless of where it was called. We cancel all child activities then set ChildHasPriority to true and IsInterruptible to false so that the Takeoff would run regardless of PickupUnit code and condition.

[abcdefg30]

Actually marking the TakeOff activity as uninterruptible doesn't seem right. When nothing inside the activity itself goes wrong, but the player decides they want to have a carryall that has descended to pick up a unit to stay landed, they cannot prevent it from taking off. That makes me think that #20403 was the incorrect fix to begin with. (And #20401 as well, unfortunately both PRs were merged with basically just one +1. The real bug is the state Land leaves us with.)
#20212 just exposed issues with the aircraft landing code which probably didn't work properly for quite some time already. I concede that fixing that is out of scope for the next release though, so I think we can take this PR.

[PunkPun]

added more documentation

[penev92]

Ok