rename mydata and/or make it customizable

[mhinsch]

I think the mydata pointer that is provided by the framework should either have a name that reflects that it is part of that framework (kmydata, kilo_data, etc.) or it should be possible to set a custom name. The default name (apart from being a bit lame) is so generic it will clash with user programs.

[fjansson]

Can it be done without breaking existing user programs and examples? If so, it's a good idea.