Self-registering IP-protocol and UDP-port dispatch; move picomap-specific constants out of generic net/dispatch layers

firmware/test.cpp

@@ -2,6 +2,7 @@
 #include "hardware/gpio.h"
 #include "dispatch.h"
 #include "handlers.h"
+#include "igmp.h"
 #include "test_handlers.h"
 
 static constexpr uint8_t LED_PIN = 25;
@@ -26,7 +27,8 @@ static constexpr handler_entry handlers[] = {
 
 int main() {
     handlers_init();
-    dispatch_init();
+    dispatch_init(PICOMAP_PORT_BE);
+    igmp::join(PICOMAP_DISCOVERY_GROUP);
     handlers_start();
 
     gpio_init(LED_PIN);