ofp-util: Rename OFPUTIL_P_* constants and update comments for clarity.

ofp-util: Rename OFPUTIL_P_* constants and update comments for clarity.

It wasn't clear to me, at least, whether an OFPUTIL_P_* constant
indicated an OpenFlow version and a flow format or just a flow format.
After some reflection, I think it's more useful if it indicates both,
because otherwise it might be necessary to pass both an OpenFlow version
and an OFPUTIL_P_* constant in some contexts, but this way only the latter
is needed.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Reviewed-by: Simon Horman <horms@verge.net.au>