The Listen Port Convention proposal should be implemented on Whonix so that server applications can easily find the proper interface to use.

• This will require a /etc/listen.conf file pointing to at least one listen.d directory with a .conf file setting the listen_ip config
• I assume we will need different files for Qubes and non-Qubes Whonix systems
• The convention is very straightforward, but I think we would still need to document how to customize those for the user's Whonix, in case they need to

This is a follow-up ticket to T635