[HACKERS] psql - add ability to test whether a variable exists

Hello,

As a follow-up to the \if patch by Corey Huinker, here is a proposal to 
allow testing whether a client-side variable exists in psql.

The syntax is as ugly as the current :'var' and :"var" things, but ISTM 
that this is the only simple option to have a working SQL-compatible 
syntax with both client-side substitution and server side execution. See 
the second example below.

   -- client side use
   psql> \set i 1
   psql> \if :?i?
   psql>   \echo 'i is defined'
   psql> \endif

   -- use server-side in an SQL expression
   psql> SELECT NOT :?VERSION_NUM? OR :'VERSION' <> VERSTION() AS bad_conf \gset
   psql> \if :bad_conf \echo 'too bad...' \quit \endif

The other option would be to have some special keyword syntax, say 
"defined var", but then it would have to be parsed client side, and how to
do that in an SQL expression is unclear, and moreover it would not look 
right in an SQL expression. If it would look like a function call, say 
"defined('var')", it would potentially interact with existing server-side 
user-defined functions, which is pretty undesirable. Hence the :?...? 
proposal above which is limited to variable subsitution syntax.

-- 
Fabien.
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