Pingsbacks should not be sent multiple times if receiving site already has it

[mdawaffe]

When WP sends a pingback to a site that already has that pingback registered, WP does not add that site to it's list of "Already Pinged" ($post->pinged). It should.

I'm not sure how edgy this is. It may happend fairly often if the original pingback was marked as spam.

[markjaquith]

(In [5772]) Mark pingback as already pinged when the target site says it already got the pingback. props mdawaffe. fixes #4289