Merged and pushed.

I'm currently preparing lab 2.0 (which removes many deprecated things). In lab 2.0 --validate 
is always prepended to the driver_options. In lab 1.x adding --validate is now recommended in 
the documentation.

No, go ahead! :-)

Florian and Silvan see no need for the extra option. Of course, you can always 
use your BDFL veto :-)

Perhaps ask the others what they prefer.

I removed that functionality to simplify the code and driver options. Since the 
call to validate is printed, users can always copy the call and add the options 
they need. If you think this is too cumbersome, I can add a --validate-options 
argument.

I see that it's no longer possible to pass options to the validator. Is this
what we want? I often want to pass -v.

I made a pull request at
https://bitbucket.org/jendrikseipp/downward/pull-requests/50.

Can you please have a look, Malte?

We want to add a "--validate" parameter to the driver script and only invoke VAL 
when the parameter is set. The "--debug" parameter should imply "--validate". lab 
should use the "--validate" parameter by default.