dired-mark-suffix: New command

dired-mark-suffix: New command

Now dired-mark-extension prepends '.' to extension when not present.
Add command dired-mark-suffix to preserve the previous
behaviour (Bug#25942).
* lisp/dired-x.el (dired-mark-suffix): New command;
mark files ending in a given suffix.
(dired--mark-suffix-interactive-spec): New defun.
(dired-mark-extension, dired-mark-suffix): Use it.
* doc/misc/dired-x.texi (Advanced Mark Commands): Update manual.
* test/lisp/dired-x-tests.el: New test suite; add test for these features.
; * etc/NEWS (Incompatible Lisp Changes in Emacs 26.1):
; Mention these changes.