Fix bug#10489: 24.0.92 `dired-do-copy' may create infinite directory hierarchy.

Fix bug#10489: 24.0.92 `dired-do-copy' may create infinite directory hierarchy.

* lisp/files.el (files-equal-p): New, simple equality check between two filename.
(file-subdir-of-p): New, Check if dir1 is subdir of dir2.
(copy-directory): Return error when trying to copy a directory on itself.
Add missing copy-contents arg to tramp handler.

* lisp/dired-aux.el (dired-copy-file-recursive): Same.
(dired-create-files): Modify destination when source is equal to dest when copying files.
Return also when dest is a subdir of source.