Re: Improvements in psql hooks for variables

So I pushed this, and the buildfarm members that are testing RedisFDW
immediately fell over:

*** /home/andrew/bf/root/HEAD/redis_fdw.build/test/expected/redis_fdw.out	2017-01-30 18:20:27.440677318 -0500
--- /home/andrew/bf/root/HEAD/redis_fdw.build/test/results/redis_fdw.out	2017-01-30 18:32:33.404677320 -0500
***************
*** 26,31 ****
--- 26,32 ----
         options (database '15', tabletype 'zset');
  -- make sure they are all empty - if any are not stop the script right now
  \set ON_ERROR_STOP
+ unrecognized value "" for "ON_ERROR_STOP": boolean expected
  do $$
    declare
      rows bigint;

======================================================================

Evidently, this test script is relying on the preceding behavior that
setting a bool variable to an empty string was equivalent to setting
it to "true".  If it's just that script I would be okay with saying
"well, it's a bug in that script" ... but I'm a bit worried that this
may be the tip of the iceberg, ie maybe a lot of people have done
things like this.  Should we reconsider the decision to reject empty
strings in ParseVariableBool?

			regards, tom lane