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.
Could $c->timeout be set to a timestamp in future when $c is constructed instead of running this calculation on each loop?