stenc should not move the tape

[jmwilson]

While investigating #27, I saw in the code if stenc can't determine the encryption status of the current block, it will start moving the tape forward until it gets a different answer, up to some limit. This can happen if the tape is positioned at a file mark or end of tape, which doesn't hold data.

Two things:

• requesting status should never change state - just report the information as-is that encryption status can't be determined
• should change "Volume encryption" to "Current block encryption" or such for correctness. The actual command in the spec is called "next block encryption status" because all it determines the encryption setting for the record at the current tape head position. Other parts of the volume could be encrypted or not.