chore(all): return concrete types, accept interfaces

- Remove exported interfaces unused locally
- Define interfaces to accept arguments
- Return concrete types, not interfaces

internal/tun/tun.go

@@ -2,13 +2,6 @@ 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)
 }