transport/{TCP,TLS}: optional IP_FREEBIND / IP_BINDANY bind socketops

Allows to bind to an address even if it is not actually (yet or ever)
configured. Fixes #238

Rationale:
https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/#whatdoesthismeanformeadeveloper

util/tcpsock/tcpsock.go

@@ -0,0 +1,27 @@
+package tcpsock
+
+import (
+	"context"
+	"net"
+	"syscall"
+)
+
+func Listen(address string, tryFreeBind bool) (*net.TCPListener, error) {
+	control := func(network, address string, c syscall.RawConn) error {
+		if tryFreeBind {
+			if err := freeBind(network, address, c); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+	var listenConfig = net.ListenConfig{
+		Control: control,
+	}
+
+	l, err := listenConfig.Listen(context.Background(), "tcp", address)
+	if err != nil {
+		return nil, err
+	}
+	return l.(*net.TCPListener), nil
+}

util/tcpsock/tcpsock_freebind_freebsd.go

@@ -0,0 +1,25 @@
+// +build freebsd
+
+package tcpsock
+
+import (
+	"fmt"
+	"syscall"
+)
+
+func freeBind(network, address string, c syscall.RawConn) error {
+	var err, sockerr error
+	err = c.Control(func(fd uintptr) {
+		if network == "tcp6" {
+			sockerr = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, syscall.IPV6_BINDANY, 1)
+		} else if network == "tcp4" {
+			sockerr = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_BINDANY, 1)
+		} else {
+			sockerr = fmt.Errorf("expecting 'tcp6' or 'tcp4', got %q", network)
+		}
+	})
+	if err != nil {
+		return err
+	}
+	return sockerr
+}

util/tcpsock/tcpsock_freebind_linux.go

@@ -0,0 +1,19 @@
+// +build linux
+
+package tcpsock
+
+import (
+	"syscall"
+)
+
+func freeBind(network, address string, c syscall.RawConn) error {
+	var err, sockerr error
+	err = c.Control(func(fd uintptr) {
+		// apparently, this works for both IPv4 and IPv6
+		sockerr = syscall.SetsockoptInt(int(fd), syscall.SOL_IP, syscall.IP_FREEBIND, 1)
+	})
+	if err != nil {
+		return err
+	}
+	return sockerr
+}

util/tcpsock/tcpsock_freebind_unsupported.go

@@ -0,0 +1,12 @@
+// +build !linux,!freebsd
+
+package tcpsock
+
+import (
+	"fmt"
+	"syscall"
+)
+
+func freeBind(network, address string, c syscall.RawConn) error {
+	return fmt.Errorf("IP_FREEBIND equivalent functionality not supported  on this platform")
+}