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`.
I suggest moving the macro from src/unix/internals.h to src/uv-common.h, then you don't have to duplicate it here.