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.

sys/sys/socketvar.h

@@ -208,6 +208,7 @@ extern struct mtx accept_mtx;
 #define	SS_ASYNC		0x0200	/* async i/o notify */
 #define	SS_ISCONFIRMING		0x0400	/* deciding to accept connection req */
 #define	SS_ISDISCONNECTED	0x2000	/* socket disconnected from peer */
+#define	SS_PROTOREF		0x400	/* strong protocol reference */
 
 /*
  * Socket state bits now stored in the socket buffer state field.