Add a convenience wrapper for spawn-with-handle-for-killing

[bblum]

Spawning a child task with the ability to kill it at any moment by using a returned 'task handle' is not directly supported, but can be emulated by spawning an extra task into the child's taskgroup (supervised or unlinked, doesn't matter), and having that task do a select2 on (a) a kill signal port with the parent on the other end, and (b) an on-exit notify_port with the child on the other end.

[alexcrichton]

Closing, we've decided to remove linked failure and kill propagation now happens at channel boundaries. You'd have to have the child task listening on a channel for a kill signal.