Never override CoreFoundations's forward handler

If an application links ObjFW and (Core)Foundation, overriding
CoreFoundation's forward handler breaks things.

Before this commit, this only worked depending on the load order: If
ObjFW was loaded first, everything was fine, as CoreFoundation would
just override ObjFW's forward handler. However, if CoreFoundation would
be loaded first, ObjFW would override CoreFoundation's forward handler
and break CoreFoundation.