Fix various common mispellings.

Mostly these are just comments but there are a few in documentation
and a handful in code and tests. Hopefully this doesn't cause too much
unnecessary pain for backpatching. I relented from some of the most
common like "thru" for that reason. The rest don't seem numerous
enough to cause problems.

Thanks to Kevin Lyda's tool https://pypi.python.org/pypi/misspellings

src/backend/storage/buffer/bufmgr.c

@@ -1973,7 +1973,7 @@ BufferSync(int flags)
 		}
 
 		/*
-		 * Measure progress independent of actualy having to flush the buffer
+		 * Measure progress independent of actually having to flush the buffer
 		 * - otherwise writing become unbalanced.
 		 */
 		ts_stat->progress += ts_stat->progress_slice;