Fix breakage of `Restart Session' in r6802. When restarting the

session, we were clearing the new session_closed flag, but failing
to clear must_close_session; with that set, the session was being
opened but immediately re-closed.

[originally from svn r6857]
[r6802 == 0dcdb6c3c19ecae20dcc6f947169b3c98bcbae9d]

windows/window.c

@@ -272,6 +272,7 @@ static void start_backend(void)
 	DeleteMenu(popup_menus[i].menu, IDM_RESTART, MF_BYCOMMAND);
     }
 
+    must_close_session = FALSE;
     session_closed = FALSE;
 }