[Typo3-dev] user management logic

[Fabian Panthen]

i needed authentication for only frontend users against an external 
system and achieved this by extending tslib_feUserAuth and overwriting 
start() and checkAuthentication(). i actually used the typo3 login box 
and inserted authenticated users on the fly into the typo3 tables so 
that they'd authenticate successfully against typo3 once my 
authentication had been successfull.