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

>
>
> I think ParseVariableBool is only likely to change to reject a NULL value
> rather than silently interpreting it as FALSE, which is the way it is in
> HEAD right now. That behavior is a leftover hack, really, and moving the
> treatment of unset values upstream seems a lot cleaner. See my draft
> patch at
> https://www.postgresql.org/message-id/30629.1485881533@sss.pgh.pa.us
>
> regards, tom lane
>

Updated patch:
- rebased on post-psql hooks master
- took nearly every suggestion for change to documentation
- \if ERROR will throw the entire \if..\endif into IGNORE mode
- state is now pushed on all \ifs
- reinstated leveraging of ParseVariableBool
- history is now kept in interactive mode regardless of \if-truth
- reworked coding example to cause less agita
- removed MainLoop "are endifs balanced" variable in favor of in-place
check which respects ON_ERROR_STOP.
- make changes to psql/Makefile to enable TAP tests and created t/ directory
- wrote an intentionally failing TAP test to see if "make check" executes
it. it does not. need help.

I'm hoping my failure in that last bit is easy to spot/fix, so I can move
forward with testing unbalanced branching, etc.