Fix preservation of the original value of PRIMARY after dropping on xterm

Fix preservation of the original value of PRIMARY after dropping on xterm

* src/xselect.c (x_own_selection): New arg `dnd_data'.  Record
it.
(x_get_local_selection, x_handle_selection_request)
(x_convert_selection): Convert the DND data instead if the
situation warrants.
(Fx_own_selection_internal, Fx_get_selection_internal)
(Fx_get_local_selection): Update calls to x_get_local_selection.

* src/xterm.c (x_dnd_do_unsupported_drop): If obtaining
selection ownership failed, return.  Record DND value and
preserve the current value of PRIMARY, if it exists.

* src/xterm.h: Update prototypes.