Further cleanup of dynahash.c API, in pursuit of portability and

readability.  Bizarre '(long *) TRUE' return convention is gone,
in favor of just raising an error internally in dynahash.c when
we detect hashtable corruption.  HashTableWalk is gone, in favor
of using hash_seq_search directly, since it had no hope of working
with non-LONGALIGNable datatypes.  Simplify some other code that was
made undesirably grotty by promixity to HashTableWalk.

src/include/lib/hasht.h

@@ -1,24 +0,0 @@
-/*-------------------------------------------------------------------------
- *
- * hasht.h
- *	  hash table related functions that are not directly supported
- *	  under utils/hash.
- *
- *
- * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
- * Portions Copyright (c) 1994, Regents of the University of California
- *
- * $Id: hasht.h,v 1.12 2001/01/24 19:43:24 momjian Exp $
- *
- *-------------------------------------------------------------------------
- */
-#ifndef HASHT_H
-#define HASHT_H
-
-#include "utils/hsearch.h"
-
-typedef void (*HashtFunc) (void *hashitem, Datum arg);
-
-extern void HashTableWalk(HTAB *hashtable, HashtFunc function, Datum arg);
-
-#endif	 /* HASHT_H */