Autosave buffers on logout if HAVE_NTGUI

Autosave buffers on logout if HAVE_NTGUI

* src/w32fns.c (w32_wnd_proc): Pass a WM_ENDSESSION message on to
w32_read_socket.

* src/w32term.c (w32_read_socket): Create an event of type
END_SESSION_EVENT if a WM_ENDSESSION message is received.

* src/termhooks.h [HAVE_NTGUI]: New event kind END_SESSION_EVENT.

* src/keyboard.c [HAVE_NTGUI] (syms_of_keyboard): New symbol
`end-session'.
(kbd_buffer_get_event): Return an end-session event if an event of
type END_SESSION_EVENT is read.
(keys_of_keyboard): Bind the end-session event to kill-emacs in
special-event-map.  (Bug#23483)