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/server.go

@@ -72,6 +72,11 @@ type Server struct {
 	// 5m is applied to help avoid resource exhaustion.
 	IdleTimeout caddy.Duration `json:"idle_timeout,omitempty"`
 
+	// KeepAliveInterval is the interval at which TCP keepalive packets
+	// are sent to keep the connection alive at the TCP layer when no other
+	// data is being transmitted. The default is 15s.
+	KeepAliveInterval caddy.Duration `json:"keepalive_interval,omitempty"`
+
 	// MaxHeaderBytes is the maximum size to parse from a client's
 	// HTTP request headers.
 	MaxHeaderBytes int `json:"max_header_bytes,omitempty"`