Kernel: PID/TID typing

This compiles, and contains exactly the same bugs as before.
The regex 'FIXME: PID/' should reveal all markers that I left behind, including:
- Incomplete conversion
- Issues or things that look fishy
- Actual bugs that will go wrong during runtime

Kernel/Syscalls/fork.cpp

@@ -95,7 +95,7 @@ pid_t Process::sys$fork(RegisterState& regs)
     }
 
     child_first_thread->set_state(Thread::State::Skip1SchedulerPass);
-    return child->pid();
+    return child->pid().value();
 }
 
 }