Deallocate PTY's when they close.

This required a fair bit of plumbing. The CharacterDevice::close() virtual
will now be closed by ~FileDescriptor(), allowing device implementations to
do custom cleanup at that point.

One big problem remains: if the master PTY is closed before the slave PTY,
we go into crashy land.

AK/Badge.h

@@ -0,0 +1,7 @@
+#pragma once
+
+template<typename T>
+class Badge {
+    friend T;
+    Badge() { }
+};