core: Refactor listeners; use SO_REUSEPORT on Unix (#4705)

* core: Refactor listeners; use SO_REUSEPORT on Unix

Just an experiment for now

* Fix lint by logging error

* TCP Keepalive configuration (#4865)

* initial attempt at TCP Keepalive configuration

* core: implement tcp-keepalive for linux

* move canSetKeepAlive interface

* Godoc for keepalive server parameter

* handle return values

* log keepalive errors

* Clean up after bad merge

* Merge in pluggable network types

From 1edc1a45e3

* Slight refactor, fix from recent merge conflict

Co-authored-by: Karmanyaah Malhotra <karmanyaah.gh@malhotra.cc>

modules/caddyhttp/app.go

@@ -384,7 +384,7 @@ func (app *App) Start() error {
 			for portOffset := uint(0); portOffset < listenAddr.PortRangeSize(); portOffset++ {
 				// create the listener for this socket
 				hostport := listenAddr.JoinHostPort(portOffset)
-				ln, err := caddy.Listen(listenAddr.Network, hostport)
+				ln, err := caddy.ListenTimeout(listenAddr.Network, hostport, time.Duration(srv.KeepAliveInterval))
 				if err != nil {
 					return fmt.Errorf("%s: listening on %s: %v", listenAddr.Network, hostport, err)
 				}