extract-ghkm: workaround for extract-parallel issue

Don't write glue grammar or unknown word label files unless the sentence
offset is 0.  This prevents multiple instances of extract-ghkm writing
to the same two files when extract-parallel is used.

TODO Better solutions might be:
 1. modify extract-parallel so that it only configures one instance of
    extract-ghkm to write the glue / unknown-lhs files (like the current
    workaround, this assumes file chunks are representative of the whole)
 2. add multithreading support directly to extract-ghkm
 3. write distinct output files for each extract-ghkm instance and
    combine them on completion