ftp: fix ls arguments for python3

ftp: fix ls arguments for python3

During the conversion to python 3, ftp ls could no longer
get option arguments, because we're now passing *bytes* to
the option parser that still expects *str*.

Fix this by fsdecode()'ing the bytes into str, which then
causes the option parsing to work properly, and the result
will be fsencode()'d again via argv_bytes() for usage.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>