Fix graceful cancellation of daemon actors

Previously, his was a bug where if the soft wait on a sub-process (the
``await .proc.wait()``) in the reaper task teardown was cancelled we
would fail over to the hard reaping sequence (meant for culling off any
potential zombies via system kill signals). The hard reap has a timeout
of 3s (currently though in theory we could make it shorter?) before
system signalling kicks in. This means that any daemon actor still
running during nursery exit would get hard reaped (3s later) instead of
cancelled via IPC message. Now we catch the ``trio.Cancelled``, call
``Portal.cancel_actor()`` on the daemon and expect the child to
self-terminate after the runtime cancels and shuts down the process.