net: implement a SO_DOMAIN getsockoption

This sockopt goes in line with SO_TYPE and SO_PROTOCOL. It makes it
possible for userspace programs to pass around file descriptors — I
am referring to arguments-to-functions, but it may even work for the
fd passing over UNIX sockets — without needing to also pass the
auxiliary information (PF_INET6/IPPROTO_TCP).

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/mips/include/asm/socket.h

@@ -43,6 +43,7 @@ To add: #define SO_REUSEPORT 0x0200	/* Allow local address and port reuse.  */
 #define SO_RCVTIMEO 	0x1006	/* receive timeout */
 #define SO_ACCEPTCONN	0x1009
 #define SO_PROTOCOL	0x1028	/* protocol type */
+#define SO_DOMAIN	0x1029	/* domain/socket family */
 
 /* linux-specific, might as well be the same as on i386 */
 #define SO_NO_CHECK	11