Shellserver shutdown capability and making Success public

[lobsterjerusalem]

This merge does the following:

• Fixes issues with the shell servers not exiting gracefully.
• Exposes Success from shell servers.

This is most useful for having a way to not only remotely trigger the shutdown of the ShellServers' respective clients/listener sockets and then knowing whether or not cleanup can happened on the basis of whether or not a client actually connected.

In order to make use of these changes the exploit must currently capture the Interrupt signal, example below.

// run exploit
	c := make(chan os.Signal, 1)
	signal.Notify(c, os.Interrupt)
	go func(){
		for range c {
				output.PrintStatus("recvd ctrl c")
			if simpleshell.GetServerInstance().Success {
				output.PrintStatus("killing shell server")
				simpleshell.GetServerInstance().KillServer()
                                doCleanup()
			}
		}
	}()

[lobsterjerusalem]

Also open to edits to internalize the Interrupt capturing, or that can be a different PR to keep things as a relevant as possible.

[terrorbyte]

Closing in favor of #358 which uses this as a basis