Resetting a single statistics counter

In the spirit of finishing off small patches, attached is the one that
implements pg_stat_reset_single(), to be able to reset stats for a
single table or function. I kind of thought it would be included in
the patch from Greg Smith for shared counters so I put it aside, but I
guess I misunderstood him there. Anyway, I polished off the final
part, and here it is.

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/