*nix fixes

I keep bundling these in with other things, and they end up not getting merged as a result of those other things, so here's a PR with just the 3 fixes that keep bugging me :P 

* Fix for a clang warning in plPythonPack about size_t comparison with -1.
* Fix for warnings about plFileSystem using mktemp.
* Fix for semaphores on *nix being used without ever being initialized.
* Fix for semaphore TryWait not working on *nix.