dunecontrol cannot handle paths with white spaces properly.

In my oppinion white spaces should not be used in paths, but if someone does, a good error message would be helpful. Running ./dune-common/bin/dunecontrol --opts=config.opts all on dune-common when located in a path that contains a white space /home/robertk/tmp/dune test gives this:

./dune-common/bin/dunecontrol: line 229: /home/robertk/tmp/dune: No such file or directory
./dune-common/bin/dunecontrol: line 225: /home/robertk/tmp/dune: No such file or directory
./dune-common/bin/dunecontrol: line 227: /home/robertk/tmp/dune: No such file or directory
./dune-common/bin/dunecontrol: line 229: /home/robertk/tmp/dune: No such file or directory
./dune-common/bin/dunecontrol: line 786: /home/robertk/tmp/dune: No such file or directory

The build then proceeds but I can imagine that the outcome might be different then expected.