Enable server-only operation and add server section to configs

- Fixed init_connections() to allow 0 connections for server-only mode
- All instances now require [server] section for bind socket
- Added test configs demonstrating server-only and client-with-server patterns
- Created run_two_instance_test.sh for manual handshake testing

src/etcp_connections.c

@@ -562,8 +562,10 @@ int init_connections(struct UTUN_INSTANCE* instance) {
         client = client->next;
     }
     
-    if (instance->connections_count == 0) {
-        DEBUG_ERROR(DEBUG_CATEGORY_ETCP, "No connections initialized");
+    // If there are clients configured but no connections created, that's an error
+    // If there are no clients (server-only mode), 0 connections is OK (server will accept incoming)
+    if (instance->connections_count == 0 && config->clients != NULL) {
+        DEBUG_ERROR(DEBUG_CATEGORY_ETCP, "Clients configured but no connections initialized");
         return -1;
     }