chore(env): PRIVATE_INTERNET_ACCESS_VPN_PORT_FORWARDING

- With retro-compatibility with `PORT_FORWARDING`

internal/configuration/sources/env/portforward.go

@@ -6,11 +6,14 @@ import (
 	"github.com/qdm12/gluetun/internal/configuration/settings"
 )
 
-func readPortForward() (
+func (r *Reader) readPortForward() (
 	portForwarding settings.PortForwarding, err error) {
-	portForwarding.Enabled, err = envToBoolPtr("PORT_FORWARDING")
+	key, _ := r.getEnvWithRetro(
+		"PRIVATE_INTERNET_ACCESS_VPN_PORT_FORWARDING",
+		"PORT_FORWARDING")
+	portForwarding.Enabled, err = envToBoolPtr(key)
 	if err != nil {
-		return portForwarding, fmt.Errorf("environment variable PORT_FORWARDING: %w", err)
+		return portForwarding, fmt.Errorf("environment variable %s: %w", key, err)
 	}
 
 	portForwarding.Filepath = envToStringPtr("PORT_FORWARDING_STATUS_FILE")