Listen on an ephemeral port

[danth]

ipget is hardcoded to use a specific port when using the temp node strategy. This causes issues with running multiple ipget instances in parallel, since only one process can use the port at a time.

The obvious, and most efficient workaround is to install a proper node and use the local node strategy instead. However, I cannot rely on this as I want to use it to fetch the dependencies of a Nix package, which must be buildable on a machine without any additional software.

To fix this, ipget should request an ephemeral port from the kernel. As a lesser solution it could try incrementing port numbers until one is available.