lib-settings, config: Override settings now ignore named (non-list) filters

lib-settings, config: Override settings now ignore named (non-list) filters

For example if configuration has:

sdbox {
  mail_path = ~/sdbox
}

Then userdb or CLI -o parameter can override mail_path without having to
specify sdbox/ prefix. This makes the behavior easier to understand and
change, especially when overriding the default settings which are inside
named filters.

This does not apply to named list filters, e.g. if you have
namespace/inbox/mail_path set, it won't be overridden with just changing
mail_path.