linux(4): Add struct clone_args for future clone3 system call.

In preparation for clone3 system call add struct clone_args and use it in
clone implementation.
Move all of clone related bits to the newly created linux_fork.h header.

Differential revision: https://reviews.freebsd.org/D31474
MFC after: 2 weeks

(cherry picked from commit 0a4b664ae8582b7596858317b53d84eb4f8bbf9a)