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/optimizer/util/tlist.c

@@ -770,7 +770,7 @@ apply_pathtarget_labeling_to_tlist(List *tlist, PathTarget *target)
  *	  the aggtranstype or aggserialtype. This allows exprType() to return the
  *	  actual type that will be produced.
  *
- * Note: We expect 'target' to be a flat target list and not have Aggrefs burried
+ * Note: We expect 'target' to be a flat target list and not have Aggrefs buried
  * within other expressions.
  */
 void