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/CharacterDevice.h

@@ -14,6 +14,7 @@ public:
     InodeMetadata metadata() const { return { }; }
 
     virtual RetainPtr<FileDescriptor> open(int& error, int options);
+    virtual void close();
 
     virtual bool can_read(Process&) const = 0;
     virtual bool can_write(Process&) const = 0;