Define two new inpcb flags in the inp_vflag field, which for whatever
reason, seems to be where new flags are getting defined:
INP_DROPPED - The protocol has terminated this connection and the socket
is not reusable: when the socket code enters the protocol, an error is immediately returned. This will substitute for NULLing the so_pcb socket field, helping to implement the invariant that all valid sockets have valid pcb's in TCP.
INP_SOCKREF - The protocol has become the owner of the socket reference,
and will need to free it when freeing the pcb, which will be used when a TCP socket is closed but still has queued data.
MFC after: 1 month