pgsql: Fix ALTER TABLE .. ENABLE/DISABLE TRIGGER recursion

Fix ALTER TABLE .. ENABLE/DISABLE TRIGGER recursion

More precisely, correctly handle the ONLY flag indicating not to
recurse. This was implemented in 86f575948c77 by recursing in
trigger.c, but that's the wrong place; use ATSimpleRecursion instead,
which behaves properly. However, because legacy inheritance has never
recursed in that situation, make sure to do that only for new-style
partitioning.

I noticed this problem while testing a fix for another bug in the
vicinity.

This has been wrong all along, so backpatch to 11.

Discussion: https://postgr.es/m/20201016235925.GA29829@alvherre.pgsql

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0e6b6f8c7192c82f62b4bbc0b40e9c6252a67bd1

Modified Files
--------------
src/backend/commands/tablecmds.c | 2 ++
src/backend/commands/trigger.c | 21 -------------
src/test/regress/expected/triggers.out | 56 ++++++++++++++++++++++++++++++++++
src/test/regress/sql/triggers.sql | 35 +++++++++++++++++++++
4 files changed, 93 insertions(+), 21 deletions(-)