socket-util: Disable dscp setting on Windows.

According to msdn documentation, one is discouraged from using
IP_TOS for ipv4 sockets (it apparently does not actually set
anything). Also, IPV6_TCLASS does not work in
Windows (it always returns an error and also is undocumented).
Looks like Microsoft recommends QoS2 APIs to achieve
the same. Till we add those API calls, simply return on Windows.

(Noticed while running unit tests for ipv6. set_dscp would fail.)

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>