$hline argument for setting the --hline parameter value. This change does
not effect any current modules as it turns out that not one single usage of
f_dialog_msgbox() relied on accepting more than a first argument (read: all
occurrences quoted the first parameter; so $* was never depended upon).
This will allow some custom invocations of --msgbox to roll over to this
function (for example, in `mouse/disable').
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