Set thread names on Linux

This is using the Linux-only prctl(PR_SET_NAME, …) call, because glibc’s pthread_setname_np() is doing exactly the same call so there is no reason to use it instead, as it isn’t any more portable.

I don’t have any other OS to test this on, but if you want to add one just add an #else defined(__YOUR_OS__) before the #else in thread.h.