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'm not sure if I like the idea that any "user input" gets copied onto $self without any kind of filtering. Can we just keep the data in $self->{opt} instead?