Fix inconsistencies and typos in the tree, take 9

This addresses more issues with code comments, variable names and
unreferenced variables.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/7ab243e0-116d-3e44-d120-76b3df7abefd@gmail.com

src/backend/commands/copy.c

@@ -176,7 +176,6 @@ typedef struct CopyStateData
 	 * Working state for COPY FROM
 	 */
 	AttrNumber	num_defaults;
-	FmgrInfo	oid_in_function;
 	FmgrInfo   *in_functions;	/* array of input functions for each attrs */
 	Oid		   *typioparams;	/* array of element types for in_functions */
 	int		   *defmap;			/* array of default att numbers */

src/backend/commands/define.c

@@ -139,7 +139,7 @@ defGetBoolean(DefElem *def)
 
 				/*
 				 * The set of strings accepted here should match up with the
-				 * grammar's opt_boolean production.
+				 * grammar's opt_boolean_or_string production.
 				 */
 				if (pg_strcasecmp(sval, "true") == 0)
 					return true;

src/backend/commands/prepare.c

@@ -143,7 +143,7 @@ PrepareQuery(PrepareStmt *stmt, const char *queryString,
 	}
 
 	/*
-	 * grammar only allows OptimizableStmt, so this check should be redundant
+	 * grammar only allows PreparableStmt, so this check should be redundant
 	 */
 	switch (query->commandType)
 	{

src/backend/commands/vacuum.c

@@ -847,7 +847,7 @@ get_all_vacuum_rels(int options)
 }
 
 /*
- * vacuum_set_xid_limits() -- compute oldest-Xmin and freeze cutoff points
+ * vacuum_set_xid_limits() -- compute oldestXmin and freeze cutoff points
  *
  * The output parameters are:
  * - oldestXmin is the cutoff value used to distinguish whether tuples are