Simplifies calling these functions, and makes code simpler

functions:
deleteUser
getUsers
userExists

- regenerate the session for every successful login
- properly destroy a session

Further information can be found on:
https://en.wikipedia.org/wiki/session_fixation

This also moves session_start in lib/base.php down a bit because we need
OC::$SERVERROOT to get the config settings.

Signed-off-by: Florian Pritz <bluewind@xinu.at>

keeping tracked of the logged in user is no longer done by the active backend but by oc_user directly instead

multiply backends can be active at the same time, allowing alternative authentication procedures like openid or tokens to be used next to the regular user system