Separate login/logout handlers required to accommodate different session lifetimes

DrupalvB fails to create a new session for a user in vBulletin upon login.
Since vBulletin supports a "remember me" option, DrupalvB needs to ensure that all cookies from vBulletin are properly deleted.
Both actions need to happen, even if a user navigates away during the request.

Committed attached patch, which adds the menu paths drupalvb/login and drupalvb/logout.

/drupalvb/login should be used as form submission target URL if login forms in vBulletin are not disabled/removed (requires customization to vBulletin's templates).

/drupalvb/logout should be used as a replacement for the regular /logout path and menu item of Drupal core. The regular logout link cannot be altered if the "Navigation" menu block is displayed on a Drupal site. In such cases, one needs to add a rewrite rule to the .htaccess file of Drupal, which redirects any requests from /logout to /drupalvb/logout.