Make Eshell globs ending in "/" match directories only

Make Eshell globs ending in "/" match directories only

* lisp/eshell/em-glob.el (eshell-glob-convert): Return whether to
match directories only.
(eshell-glob-entries): Add ONLY-DIRS argument.

* test/lisp/eshell/em-glob-tests.el
(em-glob-test/match-any-directory): New test.
(em-glob-test/match-recursive)
(em-glob-test/match-recursive-follow-symlinks): Add test cases for
when "**/" or "***/" are the last components in a glob.

* etc/NEWS: Announce this change (bug#56227).