Better abstract OFPT_SET_CONFIG and OFPT_GET_CONFIG_REPLY, make stricter.

Better abstract OFPT_SET_CONFIG and OFPT_GET_CONFIG_REPLY, make stricter.

The OFPT_SET_CONFIG and OFPT_GET_CONFIG_REPLY messages, which have the
same format, have a 'flags' field in which OpenFlow defines some bits,
which change somewhat from one version to another, and does not define
others.  Until now, Open vSwitch has not abstracted these messages at all
and has ignored the bits that OpenFlow leaves undefined.  This commit
abstracts the messages in the same way as other OpenFlow messages and
validates in OFPT_SET_CONFIG messages that the undefined bits are set to
zero.

OpenFlow 1.1 and 1.2, but not OpenFlow 1.0, define a flag named
OFPC_INVALID_TTL_TO_CONTROLLER.  Open vSwitch has until now also
implemented this as an extension to OpenFlow 1.0, and this commit retains
that extension.

Reported-by: Manpreet Singh <er.manpreet25@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Andy Zhou <azhou@ovn.org>