chore(all): return concrete types, accept interfaces

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

internal/dns/settings.go

@@ -6,22 +6,8 @@ import (
 	"github.com/qdm12/gluetun/internal/configuration/settings"
 )
 
-type SettingsGetSetter interface {
-	SettingsGetter
-	SettingsSetter
-}
-
-type SettingsGetter interface {
-	GetSettings() (settings settings.DNS)
-}
-
 func (l *Loop) GetSettings() (settings settings.DNS) { return l.state.GetSettings() }
 
-type SettingsSetter interface {
-	SetSettings(ctx context.Context, settings settings.DNS) (
-		outcome string)
-}
-
 func (l *Loop) SetSettings(ctx context.Context, settings settings.DNS) (
 	outcome string) {
 	return l.state.SetSettings(ctx, settings)