Render default DND file name handlers more precise

Render default DND file name handlers more precise

* lisp/dnd.el (dnd-protocol-alist): Redefine file name handlers
to match solely the local or remote URIs they understand.
(dnd-handle-multiple-urls): Prevent calling the same handler
multiple times for a single URI.

* lisp/gnus/mml.el (mml-dnd-protocol-alist): Apply an anologous
adjustment here.  Delete now redundant redefinition of
dnd-open-file.
(mml-dnd-attach-file): Inquire whether to apply the default
disposition and such only once even if more than one file is
dropped.

* test/lisp/dnd-tests.el (dnd-tests-receive-multiple-urls)
(dnd-tests-default-file-name-handlers): New tests.