Fixed hangs and thread memory leak in Linux

To fix issue: http://www.yafaray.org/node/318

In Linux, when using multiple threads, the missing pthread_join line caused memory leak and eventually filled up the thread stack, causing EAGAIN errors in newly created threads and therefore a complete hang of Yafaray and Blender after rendering several frames.

Even with this fix applied, there are still some other problems remaining in the code. For example, when doing photon mapping yafaray tends to crash. I have also added a change in the pthread stack size to mitigate the crashes, although it would be better to find out why the pthread stack overflows. I've also found a memory leak in the Photon Mapping integrator, and I'll make a different commit with a suggested fix for that.