Actually logs the user in to the forum

• Calls the $modSettings['integrate_login'] function if it's defined and exists
• Sets a login cookie and a session
• Logs you back out if the forum's in maintenance mode and you're not an admin
• Resets the failed login threshold
• Checks to see if you're banned
• Sets up the admin login and session if necessary
• Updates the "last login" time
• Deletes the old log_online entry for the guest