Kernel: Distinguish between new and old process groups with equal pgids

This does not add any behaviour change to the processes, but it ties a
TTY to an active process group via TIOCSPGRP, and returns the TTY to the
kernel when all processes in the process group die.
Also makes the TTY keep a link to the original controlling process' parent (for
SIGCHLD) instead of the process itself.

Kernel/Process.cpp

@@ -111,6 +111,7 @@ void Process::initialize()
 
     next_pid.store(0, AK::MemoryOrder::memory_order_release);
     g_processes = new InlineLinkedList<Process>;
+    g_process_groups = new InlineLinkedList<ProcessGroup>;
     g_hostname = new String("courage");
     g_hostname_lock = new Lock;