Fix my broken understanding of the role of the init-vector in the

Fix my broken understanding of the role of the init-vector in the
encryption process.  Essential property of the init-vector in CFB
encryption is that no two messages encrypted with the same key should
also use the same init-vector.  New design will read files encrypted
with previous scheme, but writes them using the current time as the
init-vector, and writes the init-vector into the file as part of the
header.