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.cpp

@@ -10,6 +10,10 @@ RetainPtr<FileDescriptor> CharacterDevice::open(int& error, int options)
     return VFS::the().open(*this, error, options);
 }
 
+void CharacterDevice::close()
+{
+}
+
 int CharacterDevice::ioctl(Process&, unsigned, unsigned)
 {
     return -ENOTTY;