Fix dired drag-and-drop for certain Motif programs

Fix dired drag-and-drop for certain Motif programs

* lisp/dired.el (dired-mouse-drag): Announce _DT_NETFILE in
targets list as well.
* lisp/select.el (xselect--encode-string): New arg
`prefer-string-to-c-string'.
(xselect-convert-to-filename): Convert to TEXT instead of
C_STRING, but use STRING if the type would otherwise be
C_STRING.
(xselect-dt-netfile-available-p, xselect-tt-net-file)
(xselect-convert-to-dt-netfile): New functions.
(selection-converter-alist): New selection converter.