add 'network.find_free_port' function

src/network/Network.cpp

@@ -40,6 +40,8 @@ namespace network {
         int port,
         const ServerDatagramCallback& handler
     );
+
+    int find_free_port();
 }
 
 
@@ -90,6 +92,10 @@ Server* Network::getServer(u64id_t id, bool includePrivate) const {
     return found->second.get();
 }
 
+int Network::findFreePort() const {
+    return find_free_port();
+}
+
 u64id_t Network::connectTcp(
     const std::string& address,
     int port,