Improve compliance with the XDS and XDND protocols

Improve compliance with the XDS and XDND protocols

* lisp/select.el (xselect-convert-to-text-uri-list): Return a
type of `text/uri-list' instead of STRING or C_STRING.

* lisp/x-dnd.el (x-dnd-xds-performed): New defvar.
(x-dnd-handle-direct-save): Set it to t and handle URIs with
hostnames correctly. Also return errors correctly.
(x-dnd-handle-octet-stream): New function.
(x-dnd-do-direct-save): Handle application/octet-stream, check
results.