meta-flow: Accept NXM and OXM field names, support NXM and OXM for output.

meta-flow: Accept NXM and OXM field names, support NXM and OXM for output.

This commit makes actions that accept NXM header values also accept OXM
header values and accept OXM field names where previously only NXM field
names were accepted.

This makes it possible to add new OXM fields that don't have NXM header
values, e.g. the OXM "metadata" field.

Inspired by Joe Stringer's patch:
	http://openvswitch.org/pipermail/dev/2012-June/018344.html

Reported-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Ben Pfaff <blp@nicira.com>