pgsql: Allow UNLISTEN in hot-standby mode.

Allow UNLISTEN in hot-standby mode.

Since LISTEN is (still) disallowed, UNLISTEN must be a no-op in a
hot-standby session, and so there's no harm in allowing it. This
change allows client code to not worry about whether it's connected
to a primary or standby server when performing session-state-reset
type activities. (Note that DISCARD ALL, which includes UNLISTEN,
was already allowed, making it inconsistent to reject UNLISTEN.)

Per discussion, back-patch to all supported versions.

Shay Rojansky, reviewed by Mi Tar

Discussion: https://postgr.es/m/CADT4RqCf2gA_TJtPAjnGzkC3ZiexfBZiLmA-mV66e4UyuVv8bA@mail.gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0d5b2735122557417e24a73f92a518638d2a5a32

Modified Files
--------------
doc/src/sgml/high-availability.sgml | 11 ++++++++---
src/backend/tcop/utility.c | 2 +-
src/test/regress/expected/hs_standby_allowed.out | 3 +++
src/test/regress/expected/hs_standby_disallowed.out | 4 ----
src/test/regress/sql/hs_standby_allowed.sql | 4 ++++
src/test/regress/sql/hs_standby_disallowed.sql | 2 --
6 files changed, 16 insertions(+), 10 deletions(-)