Try to address issue reported by Michael Aram: export variables sooner, when the uplevel they come from is still well known, rather than later in the rendering, when this could not be so trivial (e.g. in OO code)
Greatly reduce complexity of code required to support -export flag for ad_form:
instead of juggle variables around to mimick export_vars, just set -export flag value as a form property, then use the real export_vars on it later when form is rendered. This will support all current features (e.g. :multiple, :sign, :array) more reliably and reduce code duplication.