getpid() function

I have implemented this TODO item:

	* Add getpid() function to backend

There were a large number of pg_stat functions that access pids and
backends slots so I added it there:
	
	test=> select pg_stat_get_backend_mypid();
	 pg_stat_get_backend_mypid 
	---------------------------
	                      2757
	(1 row)

Applied.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
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