Re: \if, \elseif, \else, \endif (was Re: PSQL commands: \quit_if, \quit_unless)

I wrote:
> IIRC, I objected to putting knowledge of ConditionalStack
> into the shared psqlscan.l lexer, and I still think that would be a bad
> idea; but we need some way to get the lexer to shut that off. Probably
> the best way is to add a passthrough "void *" argument that would let the
> get_variable callback function mechanize the rule about not expanding
> in a false branch.

Here's a proposed patch that adds a passthrough of this sort.

The passthrough argument is passed only to the get_variable callback.
I dithered about whether to also pass it to the write_error callback,
but ultimately decided not to for now. Neither psql nor pgbench wants it,
and in the case of psql we'd have to invent a separate wrapper function
because we would certainly not want to change the signature of
psql_error().

Barring objection I'll push this so that Corey can rebase over it.

regards, tom lane