[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_server/cdc_rsync_server.h

@@ -43,9 +43,10 @@ class CdcRsyncServer {
   // up-to-date by checking their sizes and timestamps.
   bool CheckComponents(const std::vector<GameletComponent>& components);
 
-  // Listens to |port|, accepts a connection from the client and runs the rsync
-  // procedure.
-  absl::Status Run(int port);
+  // Listens to any available port, accepts a connection from the client and
+  // runs the rsync procedure. Prints "Port <n>: Server is listening" to stdout,
+  // so the client can retrieve the selected port.
+  absl::Status Run();
 
   // Returns the verbosity sent from the client. 0 by default.
   int GetVerbosity() const { return verbosity_; }