ad_form: completed support for -export parameter. Now the same syntax used
for "vars" argument in export_vars proc can be used. In particular
one can specify :multiple, :array and :sign flags on variables.
This change required modification to template::widget::hidden: now, when
specifying both -value and -values flags, if -values is non-empty this will
have the precedence over -value, and this widget will me multiple.
Commit on branch 5-9