ofp-util: Further abstract definitions of action properties.

ofp-util: Further abstract definitions of action properties.

This commit primarily moves the OFPAT_ACTION and NXAST_ACTION invocations
into a new file ofp-util.def.  This allows multiple places in the source to
use them.

This commit also adds a new function ofputil_action_code_from_name().
The following commit will add the first user.