* AuthPlugin added strictUserAuth() method to allow per-user override

* AuthPlugin added strictUserAuth() method to allow per-user override
  of the strict() authentication behavior.

Fixes the hole where old local passwords could still be used to log into the global account after merging.
Based on patch by Rotem Liss from http://he.wikipedia.org/wiki/%D7%9E%D7%A9%D7%AA%D7%9E%D7%A9:Rotemliss/CentralAuth#2
Changed function name from authenticateLocally() to strictUserAuth() and reversed return value to mesh a little better with strict()