When Minix is shutting down PM broadcast a KILL signal to all processes. That is not the best way to do it as it is racy and requires many asynchronous messages between PM and VFS

The goal of this project is:

• separate killing POSIX processes from the rest of the system
• shut down system processes in the right order, e.g. filesystems before the disk drivers and not the otherway around
• remove the requirement of buffering of many asynchronous messages

This change should make the shutdown always consistent and deadlock free.