Fix closing displays when preserving selections is enabled

Fix closing displays when preserving selections is enabled

* src/frame.c (delete_frame): Bind `x-auto-preserve-selections'
to nil if deleting display.
* src/xselect.c (x_clear_frame_selections): Pass original frame
to that function.
* src/xterm.c (x_preserve_selections): Fix determining the new
owner.
(syms_of_xterm): New defsym `x-auto-preserve-selections'.
* src/xterm.h: Update prototypes.