unix, windows: return error codes directly

This commit changes the libuv API to return error codes directly rather
than storing them in a loop-global field.

A code snippet like this one:

    if (uv_foo(loop) < 0) {
      uv_err_t err = uv_last_error(loop);
      fprintf(stderr, "%s\n", uv_strerror(err));
    }

Should be rewritten like this:

    int err = uv_foo(loop);
    if (err < 0)
      fprintf(stderr, "%s\n", uv_strerror(err));

The rationale for this change is that it should make creating bindings
for other languages a lot easier: dealing with struct return values is
painful with most FFIs and often downright buggy.