Reapply most of Till Adam's initial Mac OS X fixes for KSharedDataCache to KDE 4.5 (will be in KDE Platform 4.5.4).

Although I made other changes to fix the Mac OS X issue (where timeouts were not supported), it is certainly still
possible to fail to acquire a lock and so the code needs to be ready to handle that condition.

I don't know of any specific crashes this fixes, but the current situation isn't ideal.

In addition a potential crash if the evictionPolicy() is checked when the cache is unmapped is now averted.