Systemd placeholders

[MayCXC]

Assistance Disclosure

No AI was used.

Adds an interfaces subdirective to bind to implement caddyserver#7256 without changing how addresses are parsed:

example.com {
    bind 10.0.0.1 tcp4/127.0.0.1 tcp6/[::1] {
        interfaces eth0 tcp4/eth1 tcp6/eth2
        protocols h1 h2
    }
}

Only the first address is bound per prefix for interfaces assigned to a cidr. This is preferable to composing or selecting multiple addresses to emulate a bind to device syscall; a socket manager can do that natively with the overhead of opening 2^n listeners at once, or the unreliability of trying to open one of them iteratively. e.g. for linux, the user could use caddyserver#7340 with systemd.socket and BindToDevice/FreeBind.

Tests are todo, ideally we would mock the value of net.InterfaceByName and parse a caddyfile.