Fix for GitHub issue 20 : confused about signed/unsigned in strange ?: example.

The root cause was that NetESignal::dup_exr() was not copying the calculated
type (signed/unsigned) of the expression.

In passing, found and fixed a similar issue when calculating a blended value
for a constant ternary expression.

dup_expr.cc

@@ -249,6 +249,7 @@ NetESignal* NetESignal::dup_expr() const
       NetESignal*tmp = new NetESignal(net_, word_);
       ivl_assert(*this, tmp);
       tmp->expr_width(expr_width());
+      tmp->cast_signed(has_sign());
       tmp->set_line(*this);
       return tmp;
 }

eval_tree.cc

@@ -1384,6 +1384,7 @@ NetExpr*NetETernary::blended_arguments_(const NetExpr*te, const NetExpr*fe) cons
 	    if (tv == fv) val.set(idx, tv);
 	    else val.set(idx, verinum::Vx);
       }
+      val.has_sign(has_sign());
 
       if (debug_eval_tree) {
 	    cerr << get_fileline() << ": debug: Evaluate ternary with "