CREATE DATABASE: make LOCALE apply to all collation providers.

CREATE DATABASE: make LOCALE apply to all collation providers.

For CREATE DATABASE, make LOCALE parameter apply regardless of the
provider used. Also affects initdb and createdb --locale arguments.

Previously, LOCALE (and --locale) only affected the database default
collation when using the libc provider.

Discussion: https://postgr.es/m/1a63084d-221e-4075-619e-6b3e590f673e@enterprisedb.com
Reviewed-by: Peter Eisentraut