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. To separate the different fixes, I will send the additional fixes for photon mapping in a separate Pull Request after this has been merged.