Fix @* to correctly handle non-input nets.

Fix @* to correctly handle non-input nets.

@* was only expanding to input nets. nex_input() for blocks was removing
any output net that was also an input. There was also a bug in how output
nets were removed. Only outputs currently defined were removed from the
input list.

always @(*) begin
  y = a;
  z = y;
end

would report "a" as an input. While

always @(*) begin
  z = y;
  y = a;
end

would report both "a" and "y" as inputs.

To fix this all nex_inputs now take a flag that when true (the default)
correctly removes any output from the input list. Both the above cases
will now return "a". If the flag is false outputs which are also inputs
will be included in the input list. This is what the @* elaboration code
uses to get the correct sensitivity list.