[cdc_rsync] Use ephemeral port on client (#96)

Instead of calling netstat locally to find out available ports in a
tight range, call bind() with port zero to find an available ephemeral
port. This is faster and much simpler, and will eventually help
getting rid of PortManager.

Also fixes issues with running SSH commands on Windows when the remote
shell is Powershell (aka Backslash Bingo).

common/server_socket.h

@@ -30,6 +30,15 @@ class ServerSocket : public Socket {
   ServerSocket();
   ~ServerSocket();
 
+  // Returns an available ephemeral port that can be used as a listening port.
+  // Note that calling this function, followed by StartListening() or similar,
+  // is slightly racy as another process might use the port in the meantime.
+  // However, the OS usually returns ephemeral ports in a round-robin manner,
+  // and ports remain in TIME_WAIT state for a while, which may block other apps
+  // from reusing the port. Hence, the chances of races are small. Nevertheless,
+  // consider calling StartListening() with zero |port| if possible.
+  static absl::StatusOr<int> FindAvailablePort();
+
   // Starts listening for connections on |port|.
   // Passing 0 as port will bind to any available port.
   // Returns the port that was bound to.