explorer: Don't use ExitWindows on shutdown.

ExitWindows kills all current processes and that creates a race with
newly started processes. Instead, simply exit and let the new process
create a new explorer.
diff --git a/server/window.c b/server/window.c
index ca1be13..8254793 100644
--- a/server/window.c
+++ b/server/window.c
@@ -363,7 +363,7 @@
 void close_desktop_window( struct desktop *desktop )
 {
     struct window *win = desktop->top_window;
-    if (win && win->thread) post_message( win->handle, WM_SYSCOMMAND, SC_CLOSE, 0 );
+    if (win && win->thread) post_message( win->handle, WM_CLOSE, 0, 0 );
 }
 
 /* create a new window structure (note: the window is not linked in the window tree) */