allow VFS access to write stability parameters. chimera VFS always returns FILE_SYNC, as before (issue #12)

initial impl, as per https://github.com/dCache/jpnfs/issues/12
I will suggest to have add to methods fromStableHow(int)  and toStableHow()

and get rid of translateStabilityFrom/ToModel. The constants for v4 and v3 (well, yes, we break the encapsulation here,)