nixos/sslh: add transparent proxying support (#41412)

[x] Support transparent proxying. This means services behind sslh (Apache, sshd and so on) will see the external IP and ports as if the external world connected directly to them.
 [x] Run sslh daemon as unprivileged user instead of root (it is not only for security, transparent proxying requires it)
 [x] Removed pidFile support (it is not compatible with running sslh daemon as unprivileged user)
 [x] listenAddress default changed from "config.networking.hostName" (which resolves to meaningless "127.0.0.1" as with current /etc/hosts production) to "0.0.0.0" (all addresses)