Send request-pty when using exec()

There are some cases where I would like to force SSH to use a tty so that I can in turn interact with the SSH session.

This would come in handy if you want to respond to a prompt programmatically.

(e.g. sending a password, or answering yes/no to a question)

An example:

```php
<?php

$ssh = new Net_SSH2('host');
$ssh->login('username','password');

$ssh->exec('git clone git@github.com:phpseclib/phpseclib.git', false, true);  //the true would tell exec() to send request-pty

$output = '';
$complete = false;
while(!$complete) {
  $temp = $ssh->_get_channel_packet(NET_SSH2_CHANNEL_EXEC);
  if ($temp === true || $temp === false) {
    $complete = true;
  } else {
    $output .= $temp;
    if(preg_match('/\(yes\/no\)/', $temp)) {
      $ssh->_send_channel_packet(NET_SSH2_CHANNEL_EXEC, "yes\n");
    }
}
echo $output;
```

This would respond "yes" if asked about the authenticity of connecting to github.com