[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).

cdc_rsync/cdc_rsync_client.h

@@ -30,7 +30,6 @@
 
 namespace cdc_ft {
 
-class PortManager;
 class Process;
 class RemoteUtil;
 class ServerArch;
@@ -53,8 +52,6 @@ class CdcRsyncClient {
     std::string copy_dest;
     int compress_level = 6;
     int connection_timeout_sec = 10;
-    int forward_port_first = 44450;
-    int forward_port_last = 44459;
     std::string ssh_command;
     std::string sftp_command;
     std::string sources_dir;  // Base dir for files loaded for --files-from.
@@ -132,7 +129,6 @@ class CdcRsyncClient {
   const std::string destination_;
   WinProcessFactory process_factory_;
   std::unique_ptr<RemoteUtil> remote_util_;
-  std::unique_ptr<PortManager> port_manager_;
   std::unique_ptr<SocketFinalizer> socket_finalizer_;
   ClientSocket socket_;
   MessagePump message_pump_{&socket_, MessagePump::PacketReceivedDelegate()};