Split KdSendPacket and KdReceivePacket into those that manipulate the KDBG state proper (reside in kdbg/kdbg.c), and those that deal only with debug input/output that will reside in a KDTERM "KD Terminal Driver" DLL. Based on some previous preparatory work by Hervé Poussineau in PR #4600. (Equivalents of commits 5162bf106 and partly e9bcf7275.)