etcp: use human-readable names for socket type and NAT type in logs

Add helper functions server_type_str() and nat_type_str() to convert
CFG_SERVER_TYPE_* and NAT_TYPE_* codes to readable names.
Now: Add Socket type=PUBLIC instead of type=1
     self-detect check: type=PUBLIC nat=UNKNOWN instead of type=1 nat_type=0

src/transport_layer/etcp_connections.c

@@ -29,6 +29,16 @@
 #include "../lib/mem.h"
 #include "etcp.h"
 
+static const char* server_type_str(uint8_t type) {
+    static const char* names[] = {"UNKNOWN", "PUBLIC", "NAT", "PRIVATE", "LOCAL"};
+    return type < 5 ? names[type] : "?";
+}
+static const char* nat_type_str(uint8_t nat_type) {
+    if (nat_type >= 4 && nat_type <= 7) nat_type -= 4;
+    static const char* names[] = {"UNKNOWN", "EIM", "STRICT", "DIRECT"};
+    return nat_type < 4 ? names[nat_type] : "?";
+}
+
 // TCP server: on new incoming connection → create minimal ETCP_CONN
 static void tcp_server_on_link(struct stcp_link *link, void *arg) {
     struct UTUN_INSTANCE *inst = (struct UTUN_INSTANCE *)arg;
@@ -723,7 +733,7 @@ struct ETCP_SOCKET* etcp_socket_add(struct UTUN_INSTANCE* instance, struct CFG_S
         u_free(e_sock);
         return NULL;
     }
-    DEBUG_INFO(DEBUG_CATEGORY_SOCKET, "Add Socket type=%d", type);
+    DEBUG_INFO(DEBUG_CATEGORY_SOCKET, "Add Socket type=%s", server_type_str(type));
     e_sock->next = instance->etcp_sockets;
     instance->etcp_sockets = e_sock;
     e_sock->socket_id = uasync_add_socket_t(instance->ua, e_sock->fd, etcp_connections_read_callback_socket, NULL, NULL, e_sock);
@@ -1432,8 +1442,8 @@ static void send_init_response(struct ETCP_SOCKET* e_sock, struct ETCP_DGRAM* pk
         }
     }
     /* Self-detection: if my IP is non-local, I'm on a public IP */
-    DEBUG_INFO(DEBUG_CATEGORY_NAT, "self-detect check: socket=%s type=%d nat_type=%d if_addr=%s local_addr=%s",
-               e_sock->name, e_sock->type, e_sock->nat_type,
+    DEBUG_INFO(DEBUG_CATEGORY_NAT, "self-detect check: socket=%s type=%s nat=%s if_addr=%s local_addr=%s",
+               e_sock->name, server_type_str(e_sock->type), nat_type_str(e_sock->nat_type),
                sockaddr_storage_to_str(&e_sock->interface_addr).str,
                sockaddr_storage_to_str(&e_sock->local_addr).str);
     if (e_sock->nat_type != NAT_VERIFIED_DIRECT && e_sock->interface_addr.ss_family == AF_INET) {