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;

src/backend/storage/ipc/procarray.c

@@ -642,7 +642,7 @@ ProcArrayInitRecovery(TransactionId initializedUptoXID)
 	Assert(TransactionIdIsNormal(initializedUptoXID));
 
 	/*
-	 * we set latestObservedXid to the xid SUBTRANS has been initialized upto,
+	 * we set latestObservedXid to the xid SUBTRANS has been initialized up to,
 	 * so we can extend it from that point onwards in
 	 * RecordKnownAssignedTransactionIds, and when we get consistent in
 	 * ProcArrayApplyRecoveryInfo().

src/backend/storage/smgr/md.c

@@ -1788,7 +1788,7 @@ _mdfd_getseg(SMgrRelation reln, ForkNumber forknum, BlockNumber blkno,
 	BlockNumber targetseg;
 	BlockNumber nextsegno;
 
-	/* some way to handle non-existant segments needs to be specified */
+	/* some way to handle non-existent segments needs to be specified */
 	Assert(behavior &
 		   (EXTENSION_FAIL | EXTENSION_CREATE | EXTENSION_RETURN_NULL));