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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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9cb28e98b818288b70f165d08ddd295b799466c2

Modified Files
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src/backend/utils/adt/json.c | 84 +++++++++--------------------
src/test/regress/expected/json.out | 107 +++++++++++++++++++++++++++++++++++++
src/test/regress/sql/json.sql | 21 ++++++++
3 files changed, 152 insertions(+), 60 deletions(-)