chore(all): return concrete types, accept interfaces

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

internal/portforward/run.go

@@ -7,10 +7,6 @@ import (
 	"github.com/qdm12/gluetun/internal/constants"
 )
 
-type Runner interface {
-	Run(ctx context.Context, done chan<- struct{})
-}
-
 func (l *Loop) Run(ctx context.Context, done chan<- struct{}) {
 	defer close(done)