Implement -kill command

[yaroslav-gwit]

-stop is not sufficient if the underlying OS can't accept short ACPI signals (for example during a system boot, or if a VM is stuck in a reboot loop). For this purpose I'll add -kill command:

• It will ignore the machine's "online" status, and will attempt to clean up everything regardless
• After the -stop is used, new kill function will start a "process kill" loop and will prevent the VM from coming back to life (there are some edge cases, where bhyve revives the VM even after it was stopped and network interface removed, -kill will make sure VM stays down)
• Sometimes it's useful to -kill a VM instantly, avoiding the delay that is involved in a graceful shutdown

[yaroslav-gwit]

stop --force was implemented instead of kill. A bit more testing is required to confirm that it works as expected.

This issue will be closed now, but feel free to open a new one if you find any bugs in the stop --force.