Fix instanceid format to prevent session loop

instanceid is generated by uniqid() and then used as the session_name. Because session_name requires at least one letter and uniqid() does not guarantee to provide that, in the case that uniqid() generates a string of only digits every request will generate a new PHP session. This will result in either:
a) the user never being able to log in
b) if the user checks "remember me," an infinite redirection loop

I ran into this on my first trial of ownCloud today, and it had me scratching my head for quite a while.