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
the problem with that would be that, just as you said, the argument would be an int. so either i create fromStableHow3(int)/fromStableHow4(int) and risk developers picking the wrong one or i rely on the fact that the literal values are the same. which approach do you prefer?