Maint: port forwarding refactoring (#543)

- portforward package
- portforward run loop
- Less functional arguments and cycles

internal/portforward/settings.go

@@ -0,0 +1,19 @@
+package portforward
+
+import (
+	"context"
+
+	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/portforward/state"
+)
+
+type SettingsGetSetter = state.SettingsGetSetter
+
+func (l *Loop) GetSettings() (settings configuration.PortForwarding) {
+	return l.state.GetSettings()
+}
+
+func (l *Loop) SetSettings(ctx context.Context, settings configuration.PortForwarding) (
+	outcome string) {
+	return l.state.SetSettings(ctx, settings)
+}