Add corestop - a mechanism to try to stop a process instead of dumping
core, and send out a devd(8) notification. This way devd can run a script
to gather various data related to the process, or trigger a snapshot of the
VM it's running in, so that the user can later restore the snapshot, log in,
and attach to the offending proccess with a debugger.
This is a kind of WIP - it works, but might not be the final shape;
comments are welcome.