socket-util: Fix set_dscp for IPv6

socket-util: Fix set_dscp for IPv6

Try IPPROTO_IPV6/IPV6_TCLASS socket option as well as IPPROTO_IP/IP_TOS
so that this can work for IPv6 sockets.

IPPROTO_IP/IP_TOS socket option is, as it's SOL indicates, for IPv4.
What happens when it's used for IPv6 sockets?  On Linux, it seems to
be forwarded to IPv4 code and affects IPv4 part of the socket.
(e.g. non-V6ONLY case)  But it doesn't seem to be the intention of
this function.  On other platforms including NetBSD, it just fails
with ENOPROTOOPT.

Probably this function should take the address family but passing
it around lib/*stream*.c would be a bigger change.

Cc: Arun Sharma <arun.sharma@calsoftinc.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>