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

platformtest/harness/harness.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/fatih/color"
 	"github.com/pkg/errors"
+
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"

platformtest/platformtest_zpool.go

@@ -6,6 +6,7 @@ import (
 	"path/filepath"
 
 	"github.com/pkg/errors"
+
 	"github.com/zrepl/zrepl/zfs"
 )
 

platformtest/tests/undestroyableSnapshotParsing.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 
 	"github.com/stretchr/testify/require"
+
 	"github.com/zrepl/zrepl/platformtest"
 	"github.com/zrepl/zrepl/zfs"
 )