Fixed hangs and crashes 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.

Still with that fix, Photon Mapping still caused random crashes and segfaults, so I have increased the pthread stack size and it seems to have solved the issue.