Separate items by newline rather than space in drush_print_pipe

This change will have no effect on bash scripts that process the output of a drush --pipe command via backtick substitution, but it will make it much easier to process the output of some bash commands, e.g. send the output of drush pm-list --pipe for multiple sites through sort and uniq.

May affect scripts that read and process the output manually, but overall I think this is a big enough improvement to squeeze in to 3.0. If not now, then not until drush-4.