Win. Use `uv_key_t` to store current thread handle in TLS on Windows prior Vista.

When we use LoadLibrary to load libuv library process gets AV when trying assign to `uv__current_thread`.

PS. To build it also needs comment `'_WIN32_WINNT=0x0600',` line in `uv.gyp`.
What if another thread starts a new thread with uv_thread_create() right when DllMain() is called?  Is that possible?  I ask because a volatile flag doesn't seem safe in a case like that, it probably needs a mutex.