Session ID routine, modifying session module structure

I am working on adding a new member to the session module structure. It will create the session ID for the session module. It will default to NULL, in which case the session code will call its normal session_create_id function, however, if the modules define a custome session_id creation routine, it will be called instead. This will answer some issues about multiple cookies or the probability of duplicate session ids. I will post the diffs after this weekend when I can get to a real internet connection.