Implement issue #6296 passing FDs / socket activation

[MayCXC]

third time's the charm ☘️

example Caddyfile:

{
	auto_https disable_redirects
	admin off
}

http://localhost {
	bind fd/{env.CADDY_HTTP_FD} {
		protocols h1
	}
	log
	respond "Hello, HTTP!"
}

https://localhost {
	bind fd/{env.CADDY_HTTPS_FD} {
		protocols h1 h2
	}
	bind fdgram/{env.CADDY_HTTP3_FD} {
		protocols h3
	}
	log
	respond "Hello, HTTPS!"
}

as seen above, two reserved networks fd and fdgram have been added. fd/3 creates a FileListener from fd 3, fdgram/4 creates a FilePacketConn from fd 4. this simplifies configuration compared to #6543 , but the "actual" host addresses are no longer visible. so, for health checks and the admin interface, fds are treated similarly to unix sockets.

this is a good thing i think, fds should be decoupled from the address they are bound to. separate config options can be added to override the origin address for health checks / the admin interface if needed.

link to issue: #6296

[MayCXC]

this is the only significantly changed behavior from #6543 ; listenReusable locks the fd number -> file map. this is similar to what reuseUnixSocket does with unixSockets, but there are no messy race conditions involving unlink or the filesystem. it just needs to ensure that every fd is opened at most once, so the lock prevents multiple reads before the first write.

[MayCXC]

same as https://github.com/caddyserver/caddy/pull/6573/files#r1760037090