msg/simple/Accepter.cc: replace shutdown() with selfpipe event in poll() (FreeBSD)

msg/simple/Accepter.cc: replace shutdown() with selfpipe event in poll() (FreeBSD)

In FreeBSD a shutdown on the socket here can generate:
accepter.stop shutdown failed: errno 57 (57) Socket is not connected
But that does not trigger an event with the poll on the socket.

Closing the socket does.

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>