pgsql: Re-validate connection string in libpqrcv_connect().

Re-validate connection string in libpqrcv_connect().

A superuser may create a subscription with password_required=true, but
which uses a connection string without a password.

Previously, if the owner of such a subscription was changed to a
non-superuser, the non-superuser was able to utilize a password from
another source (like a password file or the PGPASSWORD environment
variable), which should not have been allowed.

This commit adds a step to re-validate the connection string before
connecting.

Reported-by: Jeff Davis
Author: Vignesh C
Reviewed-by: Peter Smith, Robert Haas, Amit Kapila
Discussion: https://www.postgresql.org/message-id/flat/e5892973ae2a80a1a3e0266806640dae3c428100.camel%40j-davis.com
Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4c03ac7e2bc46988fe4ecf3b1aef393488786f12

Modified Files
--------------
doc/src/sgml/ref/create_subscription.sgml | 11 +--
.../libpqwalreceiver/libpqwalreceiver.c | 9 +++
src/test/subscription/t/027_nosuperuser.pl | 80 ++++++++++++++++++++++
3 files changed, 95 insertions(+), 5 deletions(-)