pgsql: Replace last PushOverrideSearchPath() call with set_config_optio

Replace last PushOverrideSearchPath() call with set_config_option().

The two methods don't cooperate, so set_config_option("search_path",
...) has been ineffective under non-empty overrideStack.  This defect
enabled an attacker having database-level CREATE privilege to execute
arbitrary code as the bootstrap superuser.  While that particular attack
requires v13+ for the trusted extension attribute, other attacks are
feasible in all supported versions.

Standardize on the combination of NewGUCNestLevel() and
set_config_option("search_path", ...).  It is newer than
PushOverrideSearchPath(), more-prevalent, and has no known
disadvantages.  The "override" mechanism remains for now, for
compatibility with out-of-tree code.  Users should update such code,
which likely suffers from the same sort of vulnerability closed here.
Back-patch to v11 (all supported versions).

Alexander Lakhin.  Reported by Alexander Lakhin.

Security: CVE-2023-2454

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/dbd5795e7539ec9e15c0d4ed2d05b1b18d2a3b09

Modified Files
--------------
contrib/seg/Makefile                    |  2 +-
contrib/seg/expected/security.out       | 32 +++++++++++++++++++++++
contrib/seg/sql/security.sql            | 32 +++++++++++++++++++++++
src/backend/catalog/namespace.c         |  4 +++
src/backend/commands/schemacmds.c       | 37 +++++++++++++++++++--------
src/test/regress/expected/namespace.out | 45 +++++++++++++++++++++++++++++++++
src/test/regress/sql/namespace.sql      | 24 ++++++++++++++++++
7 files changed, 165 insertions(+), 11 deletions(-)