Maint: port forwarding refactoring (#543)

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

internal/portforward/state/startdata.go

@@ -0,0 +1,39 @@
+package state
+
+import (
+	"net"
+
+	"github.com/qdm12/gluetun/internal/provider"
+)
+
+type StartData struct {
+	PortForwarder provider.PortForwarder
+	Gateway       net.IP // needed for PIA
+	ServerName    string // needed for PIA
+	Interface     string // tun0 or wg0 for example
+}
+
+type StartDataGetterSetter interface {
+	StartDataGetter
+	StartDataSetter
+}
+
+type StartDataGetter interface {
+	GetStartData() (startData StartData)
+}
+
+func (s *State) GetStartData() (startData StartData) {
+	s.startDataMu.RLock()
+	defer s.startDataMu.RUnlock()
+	return s.startData
+}
+
+type StartDataSetter interface {
+	SetStartData(startData StartData)
+}
+
+func (s *State) SetStartData(startData StartData) {
+	s.startDataMu.Lock()
+	defer s.startDataMu.Unlock()
+	s.startData = startData
+}