Kernel: Make KBuffer a value-type wrapper around a KBufferImpl

A KBuffer always contains a valid KBufferImpl. If you need a "null"
state buffer, use Optional<KBuffer>.

This makes KBuffer very easy to work with and pass around, just like
ByteBuffer before it.

Kernel/Net/NetworkAdapter.cpp

@@ -84,7 +84,7 @@ void NetworkAdapter::did_receive(const u8* data, int length)
     m_packet_queue.append(KBuffer::copy(data, length));
 }
 
-RefPtr<KBuffer> NetworkAdapter::dequeue_packet()
+Optional<KBuffer> NetworkAdapter::dequeue_packet()
 {
     InterruptDisabler disabler;
     if (m_packet_queue.is_empty())