Implement OpenFlow 1.1+ "groups" protocol.

This doesn't include a dpif implementation of groups functionality.  In its
current form, it is untested.  Before this is committed, it needs some
more comments and an ovs-ofctl manpage update.

Signed-off-by: Neil Zhu <zhuj@centecnetworks.com>
Co-authored-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>