Make locale option behavior more consistent

Make locale option behavior more consistent

Locale options can be specified for initdb, createdb, and CREATE
DATABASE.  In initdb, it has always been possible to specify --locale
and then some --lc-* option to override a category.  CREATE DATABASE
and createdb didn't allow that, requiring either the all-categories
option or only per-category options.  In
f2553d43060edb210b36c63187d52a632448e1d2, this was changed in CREATE
DATABASE (perhaps by accident?) to be more like the initdb behavior,
but createdb still had the old behavior.

Now we change createdb to match the behavior of CREATE DATABASE and
initdb, and also update the documentation of CREATE DATABASE to match
the new behavior, which was not done in the above commit.

Author: Marina Polyakova <m.polyakova@postgrespro.ru>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://www.postgresql.org/message-id/7c99c132dc9c0ac630e0127f032ac480@postgrespro.ru