Tweak dired warning about "wildcard" characters

Tweak dired warning about "wildcard" characters

* lisp/dired-aux.el (dired-isolated-string-re): Use explicitly
numbered groups.
(dired--star-or-qmark-p): Add START parameter.  Make sure to
return the first isolated match.
(dired--need-confirm-positions, dired--mark-positions)
(dired--highlight-no-subst-chars, dired--no-subst-explain)
(dired--no-subst-ask, dired--no-subst-confirm): New functions.
(dired-do-shell-command): Use them (bug#28969, bug#35564).

* test/lisp/dired-aux-tests.el (dired-test-bug27496): Adapt to
new prompt.
(dired-test--check-highlighting): New test helper.
(dired-test-highlight-metachar): New tests.