meta-flow: Encode OXM version information into struct mf_field.

Signed-off-by: Ben Pfaff <blp@nicira.com>
meta-flow: Use OXM-defined constant for TCP flags in OpenFlow 1.5.

This also adds the definitions of a few other OXM headers we didn't have
yet.

EXT-109.
Signed-off-by: Ben Pfaff <blp@nicira.com>