pgsql: Enforce pg_class.relchecks limit (32,767 or INT16_MAX)

Enforce pg_class.relchecks limit (32,767 or INT16_MAX)

pg_class.relchecks reports the number of CHECK constraints in a table,
and is stored within a smallint. It was possible to define a table that
could overflow the counter when specifying more than 32k constraints
(new table or adding a constraint). This would cause the table to
become undroppable, as this counter should never be negative when
dropping these constraints.

Author: Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Reviewed-by: Jian He <jian(dot)universality(at)gmail(dot)com>
Reviewed-by: Sivaprasad <sivaprasad(dot)postgres(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAEze2WjSW4qaHYnvkTtbknp85eGEirGJTLMZatotdJJKiA+_hA@mail.gmail.com
Backpatch-through: 14

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b80d5b17ea3cff60370e08983546bd7b1e4298bd

Modified Files
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doc/src/sgml/limits.sgml | 7 +++++++
src/backend/catalog/heap.c | 28 +++++++++++++++++++++++-----
2 files changed, 30 insertions(+), 5 deletions(-)