Start implementation of `clone` syscall

[michalfita]

This is big task to cover all flags this syscall supports, so this is not going to end up with one issue, especially as some testing in the process would be required.

This call is used by pthread_create() in system libraries (I've checked musl source code so far), so it's crucial to support user space threading.

I would assign myself to this task, but I don't have such powers. Could we have project or milestone for multithreading?

[nuta]

I've created a milestone for multithreading.