drakshell: bootstrap agent to reduce injector usage

[psrok1]

Yeah, I know that adding agent to the agentless sandbox is a funny plot-twist, but Drakvuf Sandbox needs to make series of operations on guest before analysis has even started. My previous idea has faiiled (#933) and it seems we can't reliably hijack a thread in explorer.exe, especially for operations that may take a bit longer like file upload or sync command execution.

That's why I'm coming with another idea - injecting small agent using shellcode mode in injector that:

• spawns its own thread as soon as the code is executed
• connects to the COM1 port (which is bound to the Unix socket on the other side)
• acts like a shell, allowing to upload/download files and execute arbitrary commands on the Windows system
• disappears when no longer needed by calling VirtualFree on itself and exiting the thread

The things I don't like in this solution is that:

• we need to enable serial port device. It's not a device that can be hot-plugged so we can't reuse an existing snapshot, we need to reboot it
• it may be difficult to handle cases when we're out of sync with the agent
• it doesn't improve the injector itself
• injector -m shellcode declares that it works only on Win10 x64

[psrok1]

Well, that was interesting idea, but injector is stable enough after fixes.