Cygwin: fix facl on sockets that are not socket files

Cygwin: fix facl on sockets that are not socket files

If facl(2) is called on an AF_LOCAL or AF_UNIX socket that is not a
socket file, the current code calls fhandler_disk_file::facl in most
cases.  The latter expects to be operating on a disk file and uses the
socket's io_handle, which is not a file handle.

Fix this by calling fhandler_disk_file::facl only if the
fhandler_socket object is a file (determined by testing dev().isfs()).