Actor::by_pid: also search through the dead actors

Finally, my trick works with it. MPI ranks are not garbage collected
as soon as they end because the MPI instance keeps a reference on
them. With this, by_pid() works properly, even on dead but not
collected actors.

But this only works until the trash is emptied, obviously. This seem
to be enough for the test I wanted to get running, so I will not fix
that point tonight.