nx-match: Make more fields writable with NXAST_REG_MOVE and other actions.

Until now, fields writable by actions defined in terms of NXM field
numbers were special cases, but commit b3e9b2eda9a "ofproto: Optimize
datapath actions" had the side effect of making it easy to modify
additional fields.  This commit takes advantage of that to make
modifiable all the fields that the kernel datapath supports modifying.
We could make other fields modifiable by adding more support to the kernel
datapath.