ls: with --group-directories-first, also group symlinked dirs

ls: with --group-directories-first, also group symlinked dirs

* src/ls.c (is_linked_directory): A new function to
also consider symlinked directories.
(main): Rename check_symlink_color to check_symlink_mode,
and enable that with --group-directories-first.
(DIRFIRST_CHECK): Adjust to use is_linked_directory,
rather than just is_directory.
(gobble_file): Simplify to always update f->linkmode
if the stat() succeeds.
* tests/ls/group-dirs.sh: A new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the change in behavior.
Suggested by Amin Bandali in
https://lists.gnu.org/r/coreutils/2018-12/msg00017.html