Add a new ipxpcb flag, IPXP_SPX, which is set on ipxpcb's to mark them
as belonging to SPX. This replaces the implicit assumption that the cb
pointer for non-SPX pcb's will be NULL. This isn't required in TCP/IP
as different pcb lists are maintained for different IP protocols; IPX
stores all pcbs on the same global ipxpcb_list.

Foot provided by: gnn
MFC after: 1 month