pgsql: psql: Don't do backquote expansion in \unrestrict.

psql: Don't do backquote expansion in \unrestrict.

This oversight in commit 71ea0d6795 allows a malicious server to
inject shell commands into plain-text dump output that are run at
restore time on the machine running psql. To fix, interpret all
text after \unrestrict until the end of the line as its argument.

Reported-by: Lucas Velgus <velgusgus599(at)gmail(dot)com>
Reported-by: Filip Janus <fjanus(at)redhat(dot)com>
Reported-by: Daniel Bakker <daniel(at)jackds(dot)nl>
Author: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Reviewed-by: Noah Misch <noah(at)leadboat(dot)com>
Security: CVE-2026-18408
Backpatch-through: 14

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/71ca694c73cd1a808a393b2d5076e0518404468e
Author: Nathan Bossart <nathan(at)postgresql(dot)org>

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml | 5 +++++
src/bin/psql/command.c | 10 ++++++++--
src/bin/psql/t/001_basic.pl | 7 +++++++
3 files changed, 20 insertions(+), 2 deletions(-)