CREATE DATABASE command for non-libc providers

From the discussion here:

https://www.postgresql.org/message-id/CAFCRh--rtqbOBpJYFDmPD9kYCYxsxKpLW7LHxYMYhHXa2XoStw@mail.gmail.com

the CREATE DATABASE command has a tendency to throw errors in confusing
ways when using non-libc providers. I have attached a patch 0001 that
fixes a misleading hint, but it's still not great.

When using ICU or the builtin provider, it still requires coming up
with some valid locale name for LC_COLLATE and LC_CTYPE, even though
those have little or no effect. And because LOCALE is the fallback when
LC_COLLATE and/or LC_CTYPE are unspecified, it's confusing to the user
because they aren't even trying to specify a libc locale name at all.

The solution, as I see it, is:

* Force the environment variables LC_COLLATE=C and LC_CTYPE=C
unconditionally, and pg_perm_setlocale() them. This requires closing a
few loose ends, but it should be doable[1]. Even the libc provider uses
the "_l()" functions already, and no longer depends on setlocale().

* When datlocprovider<>'c', force datcollate and datctype to NULL.

* If the user specifies LC_CTYPE or LC_COLLATE to CREATE DATABASE, and
the provider is not libc, then ignore LC_COLLATE/LC_CTYPE and emit a
WARNING, rather than trying to set it based on LOCALE and getting an
error.

Regards,
    Jeff Davis

[1]
https://www.postgresql.org/message-id/cd3517c7-ddb8-454e-9dd5-70e3d84ff6a2%40eisentraut.org