systemd.listen placeholder

[MayCXC]

Assistance Disclosure

No AI was used.

Implements #7331 as a placeholder, bind sd/name/3 -> bind fd/{systemd.listen.name:3}. A custom network was only beneficial to implement support with a plugin, the actual behavior needed was just a fancy {env.*} that substitutes the index of a value from an array in the environment. Using one avoids complicating the config syntax and parsing, and sets us up for more systemd placeholders like {systemd.creds.*}