files: Unify clone_service_fd()

Make this function more generic: we want to do not dup not only
transport socket of parent, but just all service fds, which
were not opened at the moment of clone() in parent. But,
yes, currently it's only transport socket.

travis-ci: success for files: Unify clone_service_fd()
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>