Improve way of what URI is sent to YP

At this point the URI sent to YP servers is based on the hostname and global port setting. However this does not work with TLS enabled and may not work for more complex setups with internal-/external-split (including different hostnames).

An attribute to the <directory> tag should be added that takes the ID of a <listen-socket> on which behalf the YP submission should be made. That <listen-socket> may be type="virtual".