IOCP: named-pipe listener ignores backlog; clients don't retry ERROR_PIPE_BUSY #8

The IOCP named-pipe listener behaves as if its backlog were 1.

• Server: pipes.rs keeps a single spare instance and ignores opts.backlog.
• Client: Open::Pipe is one CreateFileW call, with no ERROR_PIPE_BUSY / WaitNamedPipeW retry.
• Effect: when two clients connect before the host services the accept, the second fails with ERROR_PIPE_BUSY. On Unix listen(backlog) is honoured, and libuv keeps several pending instances and retries.

Expected:

• Keep up to backlog pending instances.
• Clients retry ERROR_PIPE_BUSY within their deadline.
• A Windows contract test connects backlog clients before the first accept is serviced, and all of them succeed.

Found in the PR #5 integrator review (item 5).