Prepend php is_dir() functs in `ls` payload with @.

Because otherwise, a php warning is thrown when open_basedir
restriction is in effet, and the `ls` plugin cannot do its job.
Add phpserialize_recursive_dict_to_list() func

Because the phpserialize `dict_to_list()` function for
python generated variables from php is not recursive,
and have no recursive option for the moment.
Convert `cp` plugin to new API