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

transport/tcp/serve_tcp.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/transport"
+	"github.com/zrepl/zrepl/util/tcpsock"
 )
 
 type ipMapEntry struct {
@@ -44,16 +45,12 @@ func (m *ipMap) Get(ip net.IP) (string, error) {
 }
 
 func TCPListenerFactoryFromConfig(c *config.Global, in *config.TCPServe) (transport.AuthenticatedListenerFactory, error) {
-	addr, err := net.ResolveTCPAddr("tcp", in.Listen)
-	if err != nil {
-		return nil, errors.Wrap(err, "cannot parse listen address")
-	}
 	clientMap, err := ipMapFromConfig(in.Clients)
 	if err != nil {
 		return nil, errors.Wrap(err, "cannot parse client IP map")
 	}
 	lf := func() (transport.AuthenticatedListener, error) {
-		l, err := net.ListenTCP("tcp", addr)
+		l, err := tcpsock.Listen(in.Listen, in.ListenFreeBind)
 		if err != nil {
 			return nil, err
 		}