Maint: tun package to handle tun device operations

- Moved from openvpn package to tun package
- TUN check verifies Rdev value
- TUN create
- Inject as interface to main function
- Add integration test
- Clearer log message for end users if tun device does not exist
- Remove unix package (unneeded for tests)
- Remove tun file opening at the end of tun file creation
- Do not mock unix.Mkdev (no OS operation)
- Remove Tun operations from OpenVPN configurator

internal/tun/create.go

@@ -0,0 +1,38 @@
+package tun
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"golang.org/x/sys/unix"
+)
+
+type Creator interface {
+	Create(path string) error
+}
+
+var (
+	ErrMknod = errors.New("cannot create TUN device file node")
+)
+
+// Create creates a TUN device at the path specified.
+func (t *Tun) Create(path string) error {
+	parentDir := filepath.Dir(path)
+	if err := os.MkdirAll(parentDir, 0751); err != nil { //nolint:gomnd
+		return err
+	}
+
+	const (
+		major = 10
+		minor = 200
+	)
+	dev := unix.Mkdev(major, minor)
+	err := t.mknod(path, unix.S_IFCHR, int(dev))
+	if err != nil {
+		return fmt.Errorf("%w: %s", ErrMknod, err)
+	}
+
+	return nil
+}