Don't use xcache in case admin auth is enabled

As soon as admin auth is enabled in php.ini for xcache all admin operations can cause php fatal error.
This is actually a pretty bad design within XCache.

In case admin auth is enabled we simple ignore xcache.

fixes #3417 

@karlitschek backport to stable5 is necessary and I'd love to see this in 5.0.10, because this can cause update to break. (btw: first reported with 4.5 and nobody gave a  :shit: :imp: )

@icewind1991 @bartv2 @bantu @butonic @ringmaster THX