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.

Kernel/FileDescriptor.h

@@ -86,5 +86,7 @@ private:
 
     RetainPtr<FIFO> m_fifo;
     FIFO::Direction m_fifo_direction { FIFO::Neither };
+
+    bool m_closed { false };
 };