ovs-dpctl: Add mega flow support

ovs-dpctl: Add mega flow support

Added support to allow mega flow specified and displayed. ovs-dpctl tool
is mainly used as debugging tool.

This patch also implements the low level user space routines to send
and receive mega flow netlink messages. Those netlink suppor
routines are required for forthcoming user space mega flow patches.

Added a unit test to test parsing and display of mega flows.

Ethan contributed the ovs-dpctl mega flow output function.

Co-authored-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>