set conky in dock mode to be withdrawn

set conky in dock mode to be withdrawn

Withdrawn windows are those you get from applications supporting the
'-w' flag, like e.g. gkrellm or all those sweet WindowMaker dockapps
(wmcpu and Co.). In Fluxbox, these windows are drawn into the slit. Most
other window managers put them into their "taskbar", AFAIK.
This patch makes Conky act exactly the same if the following settings
are selected:

| own_window yes
| own_window_type dock