invalidate all user session cache when userdatabase is changed FS#1085

A reference file is now stored in data/cache/sessionpurge and is used to
check if user sessions are still valid.

To accomondate for slow auth backends DokuWiki caches user info for
a certain time in the user session.

darcs-hash:20080215114923-7ad00-6874d5211efce7d07e54de37244becc2387c1ba7.gz