Merge similar mutex code

The files src/fccache.c and src/fccfg.c contained very similar mutex-related code. This code is now placed in src/fcmutex.c.

I also concluded that the additional call to FcRandom during mutex creation was not required, and therefore I removed it. This simplified the logic, so that a goto could be replaced with a do-while.

Finally, there is the FC_MUTEX_IMPL_INIT macro. This macro can be used to initialize a mutex compile-time, but unfortunately not on Windows. This macro was unused and is therefore removed.