pg_createsubscriber: Fix duplicate publication name rejection.

pg_createsubscriber: Fix duplicate publication name rejection.

pg_createsubscriber rejected duplicate --publication values while parsing
command-line options, even when the duplicate names referred to
publications in different databases. Since publication names are
database-local objects, the same name is perfectly valid across multiple
databases.

This restriction was not a practical problem before commit 85ddcc2f4c,
which added support for reusing pre-existing publications. After that
change, users who have identically-named publications in multiple
databases (a common convention) could not use the feature without renaming
their publications.

The analogous restriction on --subscription names is intentionally kept as
they are reused as replication slot names, which are cluster-global, so
allowing duplicate subscription names without additional guards could
cause a slot-name collision. That work is left for a future release.

Author: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Shlok Kyal <shlok.kyal.oss@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Discussion: https://postgr.es/m/B08A7C89-B3DE-4C1D-A671-32AD8BAB7E22@gmail.com