As for `Actor.cancel()` requests, do the same for
`Actor._cancel_task()` but use `_invoke()` to ensure
correct msg transactions with caller. Don't cancel task
cancels on a cancel-all-tasks operation in attempt at
more determinism.

This is actually surprisingly easy to grok having gone through a lot of
pain understanding edge cases in the zombie lord dev branch. Basically
we just need to make sure actors are managed in a 2 step reap sequence.
In the "soft" reap phase we wait for the process to terminate on its own
concurrently with (maybe) waiting for its portal's final result (if it's
a `.run_in_actor()`). If this path is cancelled or errors, then we do
a "hard" reap where we timeout and send a signal to the proc to
terminate immediately. The only last remaining trick is to tie in the
root-is-debugger-aware logic to yet again avoid tty clobbers.

…poll time

With the new fixes to the trio spawner we can expect that both root
*and* depth > 1 nursery owning actors will now not clobber any children
that are in debug (either via breakpoint or through crashing). The tests
changed now include more checks which ensure the 2nd level parent-ish
actors also bubble up through into `pdb` and don't kill any of their
(crashed) children before they're done themselves debugging.