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/access/transam/generic_xlog.c

@@ -64,7 +64,7 @@ struct GenericXLogState
 	/*
 	 * page's images. Should be first in this struct to have MAXALIGN'ed
 	 * images addresses, because some code working with pages directly aligns
-	 * addresses, not an offsets from begining of page
+	 * addresses, not offsets from beginning of page
 	 */
 	char		images[MAX_GENERIC_XLOG_PAGES * BLCKSZ];
 	PageData	pages[MAX_GENERIC_XLOG_PAGES];