Add the ability to listen on a port

[RealOrangeOne]

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Many reverse proxies and alike don't support forwarding to sockets (eg Traefik). This adds the ability to specify a $PORT to have it listen on that instead.

Note that by default the previous behaviour is maintained, for backwards compatibility.

[FireMasterK]

I feel like this be simplified by checking whether the environment variable is set, and not having multiple nested if else / complex control flow.

I'll create a separate PR for this.

[FireMasterK]

See #3 for my solution

[RealOrangeOne]

I'm not really a Go person, hence it's a bit thrown together. Although IMO #3 is more complex and harder to read.

[FireMasterK]

IMO #3 is more complex and harder to read.

It's all because I handle the socket/ folder not existing better there, instead of expecting the net.Listen("unix", socket) to fail to go back to TCP/IP, I just check if the folder exists.