Change StopPonder() into more general StopSearch()

Change StopPonder() into more general StopSearch()

As StopPonder was only still used to unconditionally abort searches of
any kind, it is changed into a general StopSearch() routine that handles
the tasks that otherwise were performed before calling it. In particular
it refrains from stopping a search if none is going on, and conditionally
(based on its argument) sets 'searching' to idle if the move the search
will come up with has to be ignored.