Turn a few 'validnsps' static variables into locals

Turn a few 'validnsps' static variables into locals

There was no need for these to be static buffers, local variables work
just as well. I think they were marked as 'static' to imply that they
are read-only, but 'const' is more appropriate for that, so change
them to const.

To make it possible to mark the variables as 'const', also add 'const'
decorations to the transformRelOptions() signature.

Reviewed-by: Andres Freund
Discussion: https://www.postgresql.org/message-id/54c29fb0-edf2-48ea-9814-44e918bbd6e8@iki.fi