Cygwin: AF_LOCAL: fix fcntl and dup if O_PATH is set

Cygwin: AF_LOCAL: fix fcntl and dup if O_PATH is set

Make fhandler_socket_local::dup and fhandler_socket_local::fcntl (a
new method) call fhandler_base::dup and fhandler_base::fcntl if O_PATH
is set.

We're viewing the socket as a disk file here, but there's no need to
implement the actions of fhandler_disk_file::dup and
fhandler_disk_file::fcntl, which do nothing useful in this case beyond
what the fhandler_base methods do.  (The extra actions are only useful
when I/O is going to be done on the file.)