Support Windows Authentication on Install

Here's a patch.

CNR.
Updated summary.

Update remaining tasks

Added to remaining tasks

@Artusamak: Sorry mate, I probably should have put more info in the issue summary about how this works.

By default, the PHP SQLSRV driver uses Windows authentication.
Drupal passes a username and password to the connect function which makes it use SQL Server authentication.
If you use an empty string for username and password however, it falls back to Windows authentication.

Doco here:
http://msdn.microsoft.com/en-us/library/cc296198.aspx

By making the username not required in the install form (password is already not required), this setup is stored in settings.php and therefore used for every query run by Drupal.
For example, here's the settings in my instance of Drupal 7.10 running this patch and Windows authentication:

$databases = array (
  'default' =>
  array (
    'default' =>
    array (
      'database' => 'drupal',
      'username' => '',
      'password' => '',
      'host' => 'vasqld01\\sql01',
      'port' => '',
      'driver' => 'sqlsrv',
      'prefix' => '',
    ),
  ),
);

I haven't done exhaustive testing, but it works just fine for me. ;)

Edit: I did some more testing and it seems that only username needs to be empty for the driver to fall back to Windows authentication.