Cygwin: FIFO: fix clone

Cygwin: FIFO: fix clone

After copyto is called, make the new fhandler's pipe_name point to the
new fhandler's pipe_name_buf, which is a *copy* of the old fhandler's
pipe_name_buf.  Previously, get_pipe_name would return the wrong
result after a clone/dup, causing create_pipe_instance and open_pipe
to fail.

Also, stop the listen_client thread when cloning.  Otherwise the
thread can keep accepting connections that the cloned fhandler won't
know about.

Do this via a new method fhandler_fifo::stop_listen_client, extracted
from fhandler_fifo::close.