Schema-qualify unnest() in ALTER DATABASE ... RESET

Schema-qualify unnest() in ALTER DATABASE ... RESET

Commit 9df8727c5067 failed to schema-quality the unnest() call in the
query used to list the variables in ALTER DATABASE ... RESET. If there's
another unnest() function in the search_path, this could cause either
failures, or even security issues (when the tab-completion gets used by
privileged accounts).

Report and fix by Dagfinn Ilmari Mannsåker. Backpatch to 18, same as
9df8727c5067.

Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Reviewed-by: jian he <jian.universality@gmail.com>
Discussion: https://postgr.es/m/87qzyghw2x.fsf%40wibble.ilmari.org
Discussion: https://postgr.es/m/87tt4lumqz.fsf%40wibble.ilmari.org
Backpatch-through: 18