Process control in virtual system

[magicant]

• Any process (including the initial process) should be suspendable/terminatable under the control by the virtual system's executor.
• Currently, the Process instances in the virtual system do not have any connection with a Future task representing the process. There should be some interconnection: for example, when a stopping signal is sent to a process, the corresponding Future task should not be polled until a continuing signal is sent to the process.
• System.kill may suspend or terminate the calling process, so it should be async.
• Utilize the new mechanism in tests of the fg built-in etc.