nx-match: Move all knowledge of OXM/NXM here.

nx-match: Move all knowledge of OXM/NXM here.

This improves the general abstraction of OXM/NXM by eliminating direct
knowledge of it from the meta-flow code and other places.

Some function renaming might be called for; for example, mf_oxm_header()
may not be the best name now that the function is implemented within
nx-match.  However, these renamings would make this commit larger and
harder to review, so I'm postponing them.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>