Code maintenance: Unix abstraction interface

- Used for creating the tun device if it does not exist
- Mocks generated for testing

internal/unix/constants.go

@@ -0,0 +1,9 @@
+package unix
+
+import sysunix "golang.org/x/sys/unix"
+
+// Constants used for convenience so "os" does not have to be imported
+
+const (
+	S_IFCHR = sysunix.S_IFCHR
+)