Should not always unset env vars

The low level system call `sd_listen_fds` allows keeping env vars by accepting an int parameter. By the same token I would propose to at least making the unsetting of env vars optional. There are legitimate use cases that require the env vars to persist. For instance, a systemd managed service can reload itself by creating a child process for which the systemd sockets must be inherited. This child process will eventually replace the original service process and become the new main process, until then this reload process can occur again. Cloudflare has open sourced a Rust library for graceful process restart using this approach. One can read the details [here](https://blog.cloudflare.com/oxy-the-journey-of-graceful-restarts/).

Start by reading the issue's reference to the systemd sd_listen_fds API and the linked Cloudflare graceful-restart article. Inspect the crate's public socket-activation API to determine where environment variables are unset, then verify that an optional setting can preserve them for inherited child processes without changing the default behavior.

Written by the indexing model from the issue text.