[emacs/dired] use the ls-lisp ls emulation & sort directories first

alternatively, one could add GNU ls’s --group-directories-first to
dired-listing-switches, but the drawback of that would be that folder
symlinks wouldn’t be grouped together with the rest of the directories

Reported to the coreutils list:
https://lists.gnu.org/archive/html/coreutils/2018-12/msg00017.html