make formatPosixKey behave better

Take care of crazy key names better:

    scala> formatPosixKey("99_balloons")
    res4: String = _99_BALLOONS

    scala> formatPosixKey("99_balloons!#")
    res5: String = _99_BALLOONS__

    scala> formatPosixKey("ok_99_balloons")
    res6: String = OK_99_BALLOONS