Make psql's \set display variables in alphabetical order.

"\set" with no arguments displays all defined variables, but it does so
in the order that they appear in variables.c's list, which previously
was mostly creation order.  That makes the list ugly and hard to find
things in, and it exposes some psql implementation details to users.
(For instance, ordinary variables will move to the bottom of the list
if unset and set again, but variables that have hooks won't.)

Fix that by keeping the list in alphabetical order at all times, which
isn't much more complicated than breaking out of the insertion search
loops once we reach an entry that should be after the one to be inserted.

Discussion: https://postgr.es/m/31785.1485900786@sss.pgh.pa.us