Add SS_PROTOREF socket flag, which represents a strong reference by the

Add SS_PROTOREF socket flag, which represents a strong reference by the
protocol to the socket.  Normally protocol references are weak: that is,
the socket layer can tear down the socket (and hence protocol state)
when it finds convenient.  This flag will allow the protocol to
explicitly declare to the socket layer that it is maintaining a
strong reference, rather than the current implicit model associated
with so_pcb pointer values and repeated attempts to possibly free the
socket.