* lisp/dired-x.el (dired-mark-unmarked-files): Add an argument which
controls case folding for matching the regex (Bug#18716).
(dired-omit-case-fold): New variable. Defaults to `t' on case-sensitive
systems, `nil' otherwise.
(dired-mark-omitted, dired-omit-expunge): Use dired-omit-case-fold.
* doc/misc/dired-x.texi, etc/NEWS: Document dired-omit-case-fold.