SSH2: make it so negotiated algorithms can be seen before login

```
if (!$this->_connect()) {
    return false;
}
```
That'll break multi factor auth.

eg.

```
<?php
include('Net/SSH2.php');

$ssh = new Net_SSH2('www.domain.tld');
$result = $ssh->login($username, 'pass1') && $ssh->login('username', 'code1');
if (!$result) {
    exit('Login failed');
}
```

So the first login() call fails but because `_connect()` was called (and presumably returned true) `$this->bitmap |= NET_SSH2_MASK_CONSTRUCTOR` will have been set. On the second `login()` call `login()` will fail once again not because of bad auth but because `_connect()` will return false (because of `$this->bitmap & NET_SSH2_MASK_CONSTRUCTOR`).