Fix some file-modes races

Fix some file-modes races

* lisp/gnus/gnus-start.el (gnus-save-newsrc-file)
(gnus-slave-save-newsrc):
* lisp/gnus/gnus-uu.el (gnus-uu-initialize):
* lisp/gnus/mm-archive.el (mm-dissect-archive):
* lisp/gnus/mm-decode.el (mm-temp-files-delete)
(mm-display-external):
* lisp/image-dired.el (image-dired-create-thumb-1):
Use with-file-modes rather than setting the file modes later.
This fixes some race conditions where the file temporarily
has the wrong permissions.