Implement starting X Direct Save (XDS) drops

Implement starting X Direct Save (XDS) drops

* doc/lispref/frames.texi (Drag and Drop): Document new function
`dnd-direct-save'.
* etc/NEWS: Likewise.

* lisp/dnd.el (dnd-direct-save-remote-files): New defcustom.
(dnd-begin-file-drag): Implement defucstom.
(dnd-begin-drag-files): Add kill-emacs-hook after saving remote
file.
(dnd-direct-save): New function.
* lisp/x-dnd.el (x-dnd-known-types): Fix coding style.
(x-dnd-handle-drag-n-drop-event): Handle local value with
self-originating DND events.
(x-dnd-xds-current-file, x-dnd-xds-source-frame): New defvars.
(x-dnd-handle-direct-save, x-dnd-do-direct-save): New functions.

* src/xfns.c (Fx_begin_drag): Allow any atom to be used as a DND
action.
* src/xselect.c (symbol_to_x_atom): Make public.
* src/xterm.c (x_dnd_note_self_drop): Include selection local
value.
(x_ignore_errors_for_next_request): Don't assume x_error_message
is set.
* src/xterm.h: Update prototypes.