netlink-socket: add support for nl_lookup_genl_mcgroup()

netlink-socket: add support for nl_lookup_genl_mcgroup()

While we work out whether nl_sock_join_mcgroup() will be the mechanism
to support VPORT events, it is easy to add support for
nl_lookup_genl_mcgroup() and make progress on the other commands.

In this patch, we implement support for nl_lookup_genl_mcgroup() only
for the VPORT family though, which is all what dpif-linux.c needs.

Validation:
- A ported dpif-linux.c with epoll code commented out went so far as
to call dp_enumerate! DP Dump commands can be implemented next.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>