Automatically enable --web.systemd-socket when $LISTEN_FDS is set

[septatrix]

Almost two years ago (#95) exporter-toolkit added the --web.systemd-socket flag to listen on a pre-opened socket passed as a file descriptor. We have used this feature with success since then. However, having to manually set the flag when using a socket is quite cumbersome and error prone. Additionally, when the flag is set, exporters fail to start unless a socket is passed. This is a reasonable behavior as one explicitly told the exporter to use an existing socket.

However, it would be better to check the $LISTEN_FDS environment variable and automatically use a socket if that variable is set. If that variable is not set, the exporter would fall back to opening a port on its own.