[cdc_rsync] Use any available server port (#94)

Instead of calling netstat on the remote device to detect available
ports, simply call bind with port 0 to bind to any available port.
Since the port is not yet known when cdc_rsync_server.exe is called,
port forwarding needs to be started AFTER the server reports its port.

cdc_rsync/cdc_rsync_client.h

@@ -78,14 +78,9 @@ class CdcRsyncClient {
   absl::Status Run();
 
  private:
-  // Finds available local and remote ports for port forwarding.
-  // May update |server_arch| by properly detecting the architecture and retry
-  // if the architecture was guessed, i.e. if |server_arch|->IsGuess() is true.
-  absl::StatusOr<int> FindAvailablePort(ServerArch* server_arch);
-
   // Starts the server process. If the method returns a status with tag
   // |kTagDeployServer|, Run() calls DeployServer() and tries again.
-  absl::Status StartServer(int port, const ServerArch& arch);
+  absl::Status StartServer(const ServerArch& arch);
 
   // Stops the server process.
   absl::Status StopServer();
@@ -129,6 +124,9 @@ class CdcRsyncClient {
   // Stops the zstd compression stream.
   absl::Status StopCompressionStream();
 
+  // Returns true if the target is a remote target.
+  bool IsRemoteConnection() const { return remote_util_ != nullptr; }
+
   Options options_;
   std::vector<std::string> sources_;
   const std::string destination_;
@@ -143,10 +141,11 @@ class CdcRsyncClient {
   std::unique_ptr<ZstdStream> compression_stream_;
 
   std::unique_ptr<Process> server_process_;
+  std::unique_ptr<Process> port_forwarding_process_;
   std::string server_output_;  // Written in a background thread. Do not access
   std::string server_error_;   // while the server process is active.
   int server_exit_code_ = 0;
-  std::atomic_bool is_server_listening_{false};
+  std::atomic_int server_listen_port_{0};
   bool is_server_error_ = false;
 
   // All source files found on the client.