Add support for options

This patch series contains:
- generic handling of RFC 2347 options,
- support for RFC 2348 blksize option,
- support for RFC 2349 timeout option
- partial support for RFC 2349 tsize option

The only missing bit is support for the tsize option in WRQ requests.
I would name the attribute `blksize`... ;-)

The `opt` hash contains the attribute names, while `%ack` contains the option names, which are needed in the OACK packet. The original set of options remains in `rfc`. `opt` contains only those that were accepted by the server. If there was no need to translate, only one of those hashes would really be needed.