deprecate the use of LIBP2P_TCP_REUSEPORT

[marten-seemann]

Now that we have constructor options (and make them actually usable with libp2p/go-libp2p#1205), we don't need to control reuseport using environment variables any more. Just pass tcp.DisableReuseport() to the transport constructor instead.

This PR:

1. removes support for the ancient IPFS_REUSEPORT option, which was deprecated in 2018 (Deprecate IPFS_REUSEPORT, use LIBP2P_TCP_REUSEPORT #27)
2. deprecates the LIBP2P_TCP_REUSEPORT, displaying a warning when this env is set

[Stebalien]

I'd keep it, honestly. I'm usually not a fan of environment variables, but this one really is a global option. It's useful when a user has issues caused by reuseport and wants to disable it globally (even if the user's application doesn't directly support such a configuration option).

Libc has a few standard environment variables as well for similar reasons: https://www.gnu.org/software/libc/manual/html_node/Standard-Environment.html

The option is independently useful in case the application wants to manually enable/disable it. The environment variable is basically just a global override.

[marten-seemann]

I didn't realize reuseport is something that would cause problems depending on your system configuration. If that's the case, I agree that an env is appropriate.

I'll extract the removal of the IPFS_REUSEPORT flag into a separate PR.