pgsql: Ensure COPY TO on an RLS-enabled table copies no more than it sh

Ensure COPY TO on an RLS-enabled table copies no more than it should.

The COPY documentation is quite clear that "COPY relation TO" copies
rows from only the named table, not any inheritance children it may
have.  However, if you enabled row-level security on the table then
this stopped being true, because the code forgot to apply the ONLY
modifier in the "SELECT ... FROM relation" query that it constructs
in order to allow RLS predicates to be attached.  Fix that.

Report and patch by Antonin Houska (comment adjustments and test case
by me).  Back-patch to all supported branches.

Discussion: https://postgr.es/m/3472.1675251957@antos

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/59947bac7384ac56b5c95c69dee7655e2ed810df

Modified Files
--------------
src/backend/commands/copy.c               |  5 ++++-
src/backend/commands/copyto.c             |  4 ++--
src/test/regress/expected/rowsecurity.out | 37 +++++++++++++++++++++++++++++++
src/test/regress/sql/rowsecurity.sql      | 34 ++++++++++++++++++++++++++++
4 files changed, 77 insertions(+), 3 deletions(-)