Fixes for semaphores on *nix.
Remove plFileSystem::GetTempFileName.

It was unused and on *nix used the deprecated mktemp function.
Fix a clang warning in plPythonPack.
Merge pull request #393 from dpogue/nix-fix

*nix fixes
Use std::atomic_int for hsSafeRefCnt, and move it to hsRefCnt.h/cpp
Replace hsMutex with std::mutex
Replace hsSleep with std::thread::sleep_for
Use C++11 threading for hsEvent, and clean up the API a bit
Convert hsThread to a std::thread implementation
Merge pnUtRef with hsRefCnt, and remove the former