core: add iface as network type

[mohammed90]

This PR allows using the interface name as the network address host when using the iface+?{udp,tcp} network. In other words, user will be able to say:

example.com {
    bind iface/eth0
    respond "What's up, Doc?"
}

Because an interface may have multiple IPv6 addresses, and I can't come up with a clean up to communicate this using the host part without it being hairy, I limited this to picking up only the IPv4 address of the defined interface.

I tried at first using the format, e.g. iface/en0+tcp, for the protocol selection, but the HTTP3 listener is created by looking up a custom registered network, then using that network to lookup its listener constructor. This makes registering a custom network for UDP the only possible approach to hook our constructor, i.e. doing iface+udp et al.

I'd classify this highly experimental. It's extremely limited for only supporting IPv4 addresses. I don't foresee any possibility of adding IPv6 support without it being convolute and painful. Do we really want this?

[mholt]

This is interesting, but it's a weird violation of abstractions. The bind directive typically describes the socket not the interface. Hence you can bind to tcp/udp/unix and an address, but specifying an interface is like a whole separate thing.

I'm not sure yet what the best solution is... would be nice to avoid funky mini-syntax here.

[mohammed90]

I put this together as PoC in response to #6504 (comment). I don't know if it's the best approach or not. It's an experiment.

[MayCXC]

8 years of discussion about the interface/hostname/ipv6 multicast situation here golang/go#9334
we should make a MultiListener implementation package, add it to Caddy as a PoC, and then rally to get it added into net.go 💪