Drain buffered output on Windows after cancellation

When a child exits, the run cancels its pending I/O. On Windows, a read
issued after that cancellation returned `nil` without consuming bytes
already buffered in the pipe, dropping output the child had already
written. The kqueue and epoll backends drain the descriptor on this
path; the IOCP backend did not.

`registerHandle()` now surfaces its registration outcome so `read()`
detects the cancelled state before issuing any I/O and drains the buffer
itself. `write()` reports a zero-length write on the same path. Both
previously issued an overlapped operation and returned without awaiting
it, leaving the kernel to write into a buffer the call had already
released.

Author: broken-circle