The thread ID is an unsigned long so it can hold pthread_t so people can do naughty things with it.

I'm going to be adding additional useful thread API functions, but this should prevent crashes in people's existing code on 64-bit architectures.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%404327