make --robot and --robot-paths pathlist-typed options, update with vsc-base

@stdweird: please review
I won't do this here, but I've opened an issue for it (see #1097). This needs to be done with care, and can be tackled together with making the test suite a bit more robust (cfr. https://github.com/hpcugent/easybuild/issues/32#issuecomment-55447699)