Maint: port forwarding refactoring (#543)

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

internal/portforward/get.go

@@ -0,0 +1,9 @@
+package portforward
+
+import "github.com/qdm12/gluetun/internal/portforward/state"
+
+type Getter = state.PortForwardedGetter
+
+func (l *Loop) GetPortForwarded() (port uint16) {
+	return l.state.GetPortForwarded()
+}