ofp-prop: New module for working with OpenFlow 1.3+ properties.

ofp-prop: New module for working with OpenFlow 1.3+ properties.

Several OpenFlow 1.3+ messages use TLV-based properties that take a
common form.  Until now, ofp-util has had some static functions for
dealing with properties.  Because properties will start to be needed
outside of ofp-util, this commit breaks them out into a new library,
renaming them to begin with ofpprop_.

The following commit will add a few new interfaces that add new
functionality.

Signed-off-by: Ben Pfaff <blp@ovn.org>