Comply with the Motif requirement for unique drag atoms

Comply with the Motif requirement for unique drag atoms

* src/xselect.c (x_handle_selection_request)
(Fx_get_selection_internal, syms_of_xselect): New variable
`x-selection-alias-alist'.  Respect that alist of aliases.

* src/xterm.c (x_atom_refs): Intern _EMACS_DRAG_ATOM.
(xm_get_drag_atom_1, xm_get_drag_atom): New functions.
(xm_setup_drag_info, x_dnd_cleanup_drag_and_drop)
(x_dnd_begin_drag_and_drop, x_dnd_update_state, handle_one_xevent)
(x_connection_closed, x_intern_cached_atom): Alias the drag atom
to XdndSelection.  Use it instead of XdndSelection to set the
Motif index atom.
(x_get_atom_name): Handle new atoms.
(syms_of_xterm): New defsym.
* src/xterm.h (struct x_display_info): New fields for new atoms
and their names.