[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/params.cc

@@ -80,8 +80,6 @@ Options:
     --sftp-command <cmd>    Path and arguments of sftp command to use, e.g.
                             "C:\path\to\sftp.exe -P 12345 -i id_rsa -oUserKnownHostsFile=known_hosts"
                             Can also be specified by the CDC_SFTP_COMMAND environment variable.
-    --forward-port <port>   TCP port or range used for SSH port forwarding (default: 44450-44459).
-                            If a range is specified, searches for available ports (slower).
 -h, --help                  Help for cdc_rsync
 )";
 
@@ -305,15 +303,9 @@ OptionResult HandleParameter(const std::string& key, const char* value,
   }
 
   if (key == "forward-port") {
-    if (!ValidateValue(key, value)) return OptionResult::kError;
-    uint16_t first, last;
-    if (!port_range::Parse(value, &first, &last)) {
-      PrintError("Failed to parse %s=%s, expected <port> or <port1>-<port2>",
-                 key, value);
-      return OptionResult::kError;
-    }
-    params->options.forward_port_first = first;
-    params->options.forward_port_last = last;
+    // This param is no longer needed. Just print a warning for backwards
+    // compatibility.
+    std::cout << "--forward-port argument no longer needed" << std::endl;
     return OptionResult::kConsumedKeyValue;
   }