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

firmware/firmware.cpp

@@ -1,5 +1,6 @@
 #include "dispatch.h"
 #include "handlers.h"
+#include "igmp.h"
 
 std::string_view firmware_name = "picomap";
 
@@ -14,7 +15,8 @@ static constexpr handler_entry handlers[] = {
 
 int main() {
     handlers_init();
-    dispatch_init();
+    dispatch_init(PICOMAP_PORT_BE);
+    igmp::join(PICOMAP_DISCOVERY_GROUP);
     handlers_start();
     dispatch_run(handlers);
 }