kernel: process: implement POSIX process groups

[d4ilyrun]

Process groups

Note

We don't support POSIX's concept of terminals yet, so the parts about the session's controlling terminal are left to be implemented later.

POSIX.1-2024

3.280 Process Group

A collection of processes that permits the signaling of related processes. Each process in the system is a member of a process group that is identified by a process group ID. A newly created process joins the process group of its creator.

3.329 Session

A collection of process groups established for job control purposes. Each process group is a member of a session. A process is considered to be a member of the session of which its process group is a member. A newly created process joins the session of its creator. A process can alter its session membership; see setsid(). There can be multiple process groups in the same session

Syscalls

• setsgid, getpsid: add syscall
• setpgid, getpgid: add syscall
• waitpid: support negative values for the pid argument
• kill: support negative values for the pid argument