Respect test function when performing local drag-and-drop

Respect test function when performing local drag-and-drop

* lisp/x-dnd.el (x-dnd-test-function): Fix doc string to
describe what is actually accepted.
(x-dnd-known-types, x-dnd-targets-list): Fix coding style.
(x-dnd-handle-native-drop): New function.

* src/xselect.c (x_atom_to_symbol): Export.

* src/xterm.c (x_dnd_note_self_drop): Call new variable to
determine what action to return.
(x_clear_dnd_action): New function.
(x_dnd_begin_drag_and_drop): Respect new variable.
(syms_of_xterm): New defvar `x-dnd-native-test-function'.
* src/xterm.h: Update prototypes.