Maint: port forwarding refactoring (#543)

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

internal/portforward/state/portforwarded.go

@@ -0,0 +1,26 @@
+package state
+
+type PortForwardedGetterSetter interface {
+	PortForwardedGetter
+	SetPortForwarded(port uint16)
+}
+
+type PortForwardedGetter interface {
+	GetPortForwarded() (port uint16)
+}
+
+// GetPortForwarded is used by the control HTTP server
+// to obtain the port currently forwarded.
+func (s *State) GetPortForwarded() (port uint16) {
+	s.portForwardedMu.RLock()
+	defer s.portForwardedMu.RUnlock()
+	return s.portForwarded
+}
+
+// SetPortForwarded is only used from within the OpenVPN loop
+// to set the port forwarded.
+func (s *State) SetPortForwarded(port uint16) {
+	s.portForwardedMu.Lock()
+	defer s.portForwardedMu.Unlock()
+	s.portForwarded = port
+}