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

include/udp.h

@@ -36,6 +36,7 @@ void prepend(Buf& buf, const eth::mac_addr& dst_mac, const eth::mac_addr& src_ma
     ipv4::prepend(buf, dst_mac, src_mac, src_ip, dst_ip, 17, sizeof(header) + payload_len, ttl);
 }
 
+void init();
 void handle(std::span<const uint8_t> frame, span_writer& tx);
 
 using port_handler = void (*)(std::span<const uint8_t> payload, const address& from);