Improve compatibility with some clients of the Motif drop protocol

Improve compatibility with some clients of the Motif drop protocol

* lisp/select.el (x-dnd-targets-list): New defvar.
(xselect-convert-to-targets): Convert XdndSelection based on the
DND targets list.
* src/xfns.c (Fx_begin_drag): Pass new argument.
* src/xselect.c (struct x_selection_request): New struct.
(x_push_current_selection_request):
(x_pop_current_selection_request): New functions.
(x_selection_request_lisp_error, x_reply_selection_request)
(x_handle_selection_request, x_convert_selection)
(syms_of_xselect_for_pdumper): Correctly handle recursive
requests for MULTIPLE by maintaining a stack of selection
requests, converted selections, and other data.
* src/xterm.c (x_dnd_begin_drag_and_drop): New argument
`selection_target_list'.  Bind it to the DND targets list.
(syms_of_xterm): New defvar and associated defsym.
* src/xterm.h: Update prototypes.