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

include/igmp.h

@@ -17,6 +17,8 @@ struct __attribute__((packed)) message {
 };
 static_assert(sizeof(message) == 8);
 
+void init();
+
 eth::mac_addr mac_for_ip(const ipv4::ip4_addr& group);
 bool is_member(const ipv4::ip4_addr& ip);
 bool is_member_mac(const eth::mac_addr& mac);