Assorted minor refactoring in EXPLAIN.

This is believed to not change the output at all, with one known exception:
"Subquery Scan foo" becomes "Subquery Scan on foo".  (We can fix that if
anyone complains, but it would be a wart, because the old code was clearly
inconsistent.)  The main intention is to remove duplicate coding and
provide a cleaner base for subsequent EXPLAIN patching.

Robert Haas

src/include/lib/stringinfo.h

@@ -10,7 +10,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/lib/stringinfo.h,v 1.36 2009/01/01 17:23:59 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/lib/stringinfo.h,v 1.37 2009/07/24 21:08:42 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -131,6 +131,12 @@ extern void appendStringInfoChar(StringInfo str, char ch);
 	 appendStringInfoChar(str, ch) : \
 	 (void)((str)->data[(str)->len] = (ch), (str)->data[++(str)->len] = '\0'))
 
+/*------------------------
+ * appendStringInfoSpaces
+ * Append a given number of spaces to str.
+ */
+extern void appendStringInfoSpaces(StringInfo str, int count);
+
 /*------------------------
  * appendBinaryStringInfo
  * Append arbitrary binary data to a StringInfo, allocating more space