This function sets the filename for a custom
basefile to be searched on subsequent calls to load_dialog. By
default a basefile named "base.dcl" is searched. If init_dialog is
called with empty argument (nil or empty string), or without
argument, subsequent calls to load_dialog will not search for a
custom base-file and instead use the builtin base definitions. This
function is useful to avoid the loading of a file named base.dcl
that might be in the support search path accidentally (eg as part
of some installed CAD application).