Link refactor (#941)

* Link refactoring

* More refactoring

* More tweaking

* Cleaner shutdowns, UNIX socket support, more tweaks

* Actorise links, remove mutex

* SOCKS support

src/core/link_tcp_other.go

@@ -0,0 +1,18 @@
+//go:build !darwin && !linux
+// +build !darwin,!linux
+
+package core
+
+import (
+	"syscall"
+)
+
+// WARNING: This context is used both by net.Dialer and net.Listen in tcp.go
+
+func (t *linkTCP) tcpContext(network, address string, c syscall.RawConn) error {
+	return nil
+}
+
+func (t *linkTCP) getControl(sintf string) func(string, string, syscall.RawConn) error {
+	return t.tcpContext
+}