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>
diff --git a/arch/parisc/include/asm/socket.h b/arch/parisc/include/asm/socket.h
index a658b09..960b1e5 100644
--- a/arch/parisc/include/asm/socket.h
+++ b/arch/parisc/include/asm/socket.h
@@ -25,6 +25,7 @@
 #define SO_ERROR	0x1007
 #define SO_TYPE		0x1008
 #define SO_PROTOCOL	0x1028
+#define SO_DOMAIN	0x1029
 #define SO_PEERNAME	0x2000
 
 #define SO_NO_CHECK	0x400b