pgsql: Ignore attempts to \gset into specially treated variables.

Ignore attempts to \gset into specially treated variables.

If an interactive psql session used \gset when querying a compromised
server, the attacker could execute arbitrary code as the operating
system account running psql. Using a prefix not found among specially
treated variables, e.g. every lowercase string, precluded the attack.
Fix by issuing a warning and setting no variable for the column in
question. Users wanting the old behavior can use a prefix and then a
meta-command like "\set HISTSIZE :prefix_HISTSIZE". Back-patch to 9.5
(all supported versions).

Reviewed by Robert Haas. Reported by Nick Cleaton.

Security: CVE-2020-25696

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7b356c78f67099db0bb5dc20663adcedc273c7a0

Modified Files
--------------
src/bin/psql/common.c | 7 +++++++
src/bin/psql/variables.c | 26 ++++++++++++++++++++++++++
src/bin/psql/variables.h | 1 +
src/test/regress/expected/psql.out | 4 ++++
src/test/regress/sql/psql.sql | 3 +++
5 files changed, 41 insertions(+)