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/tun.go

@@ -0,0 +1,20 @@
+package tun
+
+import "golang.org/x/sys/unix"
+
+var _ Interface = (*Tun)(nil)
+
+type Interface interface {
+	Checker
+	Creator
+}
+
+type Tun struct {
+	mknod func(path string, mode uint32, dev int) (err error)
+}
+
+func New() *Tun {
+	return &Tun{
+		mknod: unix.Mknod,
+	}
+}