extract icmp into its own static library; replace all __attribute__((constructor)) self-registrations with explicit init() calls in dispatch_init

arp/arp.cpp

@@ -41,10 +41,7 @@ void handle(std::span<const uint8_t> frame, span_writer& tx) {
     eth::send_raw(buf.span());
 }
 
-void init() {}
-
-__attribute__((constructor))
-static void register_ethertype() {
+void init() {
     eth::register_ethertype(eth::ETH_ARP, handle);
 }