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.
The default is wrong (copy-pasta).

I used `opt` because `rfc` was short too. :-)