MTU: Support obtaining the MTU using the network interface index.

[vnt-dev]

[mxinden]

Assuming this wasn't intentional, thus closing.

@vnt-dev in case you have any questions, feel free to open a new issue.

Edit: Misread the title. Sorry. Re-opening.

[vnt-dev]

Can the API be refined to support obtaining the MTU using various methods such as IP, network interface index, or network interface name?

[larseggert]

It could, but for our use case in neqo, that's not needed.

I'm not opposed to seeing this added, but we'd need to see a PR to judge how much code this would add that would need to be maintained.