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

eth/eth.cpp

@@ -61,11 +61,6 @@ void process_frame(std::span<const uint8_t> frame, span_writer& tx) {
     }
 }
 
-__attribute__((constructor))
-void register_default_mac_filter() {
-    register_mac_filter(default_mac_filter);
-}
-
 } // namespace
 
 void register_ethertype(uint16_t ethertype_be, ethertype_handler fn) {
@@ -114,6 +109,8 @@ bool init() {
     w6300::open_socket(raw_socket, w6300::protocol::macraw, w6300::sock_flag::none);
     w6300::set_interrupt_mask(w6300::ik_sock_0);
 
+    register_mac_filter(default_mac_filter);
+
     return true;
 }