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 guess you kind of answer my previous question here. I would like if $self->{opt} didn't have any translation (blksize vs blocksize), I think I would rather have sub blocksize { shift->opt->{blksize}; } to make them friendly instead. Also, why not "has options" instead of just "opt"?