fix utility module dircolors bug when launching zsh if SHELL is csh

If the user's SHELL is a csh derivative, then the call to 'dircolors' in modules/utility/init.zsh when starting 'zsh' from csh returns a csh-style result with 'setenv', which of course causes a prezto init error. This tweak to include the '-b' option when calling dircolors ensures that code for Bourne shell and friends is returned instead.