Switch MultiHandleWait and port relay from WaitForMultipleObjects to IO completion ports

Replace the WaitForMultipleObjects-based wait loop in relay::MultiHandleWait
with an IO completion port (IOCP), removing the MAXIMUM_WAIT_OBJECTS (64)
handle limit for all callers.

Changes to relay::MultiHandleWait (relay.hpp/cpp):
- Add unique_registered_wait RAII type for RegisterWaitForSingleObject handles
- Add Register() pure virtual to OverlappedIOHandle, implemented by all subclasses
- Handles that support IOCP (pipes, sockets) register directly; handles that
  don't (console handles, events) use a RegisterWaitForSingleObject bridge
- Convert Run() to use GetQueuedCompletionStatus instead of WaitForMultipleObjects
- Cancel() posts a key=0 completion to wake Run(); key=0 sets m_cancel on dequeue
- Fix member order: m_iocp declared before m_handles for correct destruction

Changes to port relay (localhost.cpp):
- Replace AcceptThread's WaitForMultipleObjects with a direct IOCP: associate
  listen sockets via CreateIoCompletionPort, AcceptEx completions go straight
  to the IOCP, stop signal via PostQueuedCompletionStatus key=0
- Remove the MAXIMUM_WAIT_OBJECTS port limit check
- Remove AcceptEvent from PortRelay (no longer needed with direct IOCP)

Author: Ben Hillis