Move necessary headers into api/headers

The DFError.h, DFStorage.h, and DFXMLForward.h files all need to go into
the public API headers directory, as they are needed by other headers in
that directory.
Add dfconvert

This program provides command-line access to the get, put, and create
operations that were previously accessible via dfutil. It is the first
consumer to stricly use only the public API exposed by DocFormats.