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/CMakeLists.txt

@@ -82,6 +82,7 @@ set(KERNEL_SOURCES
     PCI/MMIOAccess.cpp
     PerformanceEventBuffer.cpp
     Process.cpp
+    ProcessGroup.cpp
     Profiling.cpp
     Ptrace.cpp
     RTC.cpp