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/utils/adt/formatting.c

@@ -3146,15 +3146,15 @@ DCH_from_char(FormatNode *node, char *in, TmFromChar *out)
 				{
 					int			matched,
 								years,
-								millenia,
+								millennia,
 								nch;
 
-					matched = sscanf(s, "%d,%03d%n", &millenia, &years, &nch);
+					matched = sscanf(s, "%d,%03d%n", &millennia, &years, &nch);
 					if (matched < 2)
 						ereport(ERROR,
 								(errcode(ERRCODE_INVALID_DATETIME_FORMAT),
 							  errmsg("invalid input string for \"Y,YYY\"")));
-					years += (millenia * 1000);
+					years += (millennia * 1000);
 					from_char_set_int(&out->year, years, n);
 					out->yysz = 4;
 					s += nch;

src/backend/utils/adt/geo_spgist.c

@@ -45,7 +45,7 @@
  *	(3) the prefix of the current node
  *
  * If we visualize them on our simplified drawing (see the drawing above);
- * transfered boundaries of (1) would be the outer axis, relevant part
+ * transferred boundaries of (1) would be the outer axis, relevant part
  * of (2) would be the up right part of the other axis, and (3) would be
  * the inner axis.
  *