Don't add extra newline on --pipe

We add an extra newline at the end of pipe'd output, which seems fine, but then we pass it to drush_print_r which adds another newline.

#1659668: variable-get outputs in single line when using --pipe introduced a test failure because we do an assertEmpty on a --pipe'd output that should be empty, but it fails because it's \n instead of empty.

This also creates what I consider a bug when you do something like:

drush pml --status=enabled --pipe|wc -l

That'll tell you you have one more module enabled than your actually do.

I think the best fix is to give drush_print_r the same newline option as drush_print, and set it to false when we print piped output.