CVE-2020-25717: s3:auth: no longer let check_account() autocreate local users

CVE-2020-25717: s3:auth: no longer let check_account() autocreate local users

So far we autocreated local user accounts based on just the
account_name (just ignoring any domain part).

This only happens via a possible 'add user script',
which is not typically defined on domain members
and on NT4 DCs local users already exist in the
local passdb anyway.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14556
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14801

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>