add fork support, like linux kernel, every process has one freebsd struct thread

[liujinhui-job]

The current fork implementation has an issue: after creating a socket and invoking fork, both the parent and child processes call close. When the parent process closes the socket, it is released immediately. Subsequently, when the child process attempts to close it, the file descriptor (fd) no longer exists. This behavior differs significantly from Linux, which can lead to various problems. In a true Linux implementation:

Child processes ‌inherit‌ open file descriptors from the parent upon fork, meaning the reference count of the file is incremented.
Each process in Linux corresponds to a task_struct.

The purpose of this commit is to ‌simulate Linux's behavior‌ by addressing these two aspects.

The partial reason has been explained in my CSDN blog：
https://blog.csdn.net/weixin_41607301/article/details/146396412?spm=1001.2014.3001.5502