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.