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/VirtualFileSystem.h

@@ -72,6 +72,7 @@ public:
     bool chmod(const String& path, mode_t, Inode& base, int& error);
 
     void register_character_device(CharacterDevice&);
+    void unregister_character_device(CharacterDevice&);
 
     size_t mount_count() const { return m_mounts.size(); }
     void for_each_mount(Function<void(const Mount&)>) const;