check_name_lower() is the only user of check_name_str() using
permit_upper=False.  Move the associated code from check_name_str() to
check_name_lower(), and drop the parameter.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210323094025.3569441-12-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>