pgsql: Process variadic arguments consistently in json functions

Process variadic arguments consistently in json functions

json_build_object and json_build_array and the jsonb equivalents did not
correctly process explicit VARIADIC arguments. They are modified to use
the new extract_variadic_args() utility function which abstracts away
the details of the call method.

Michael Paquier, reviewed by Tom Lane and Dmitry Dolgov.

Backpatch to 9.5 for the jsonb fixes and 9.4 for the json fixes, as
that's where they originated.

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/06d5ba0d49d2ad756c14cad1d1281e8f8092d0b1

Modified Files
--------------
src/backend/utils/adt/json.c | 84 ++++++++--------------------
src/backend/utils/adt/jsonb.c | 99 ++++++++++-----------------------
src/test/regress/expected/json.out | 107 ++++++++++++++++++++++++++++++++++++
src/test/regress/expected/jsonb.out | 105 +++++++++++++++++++++++++++++++++++
src/test/regress/sql/json.sql | 21 +++++++
src/test/regress/sql/jsonb.sql | 22 +++++++-
6 files changed, 306 insertions(+), 132 deletions(-)