Add support for unconnected_drive.

Icarus has recognized this directive, but it did not do anything
with the information. This patch adds the functionality for most
common cases. It adds this by changing the input net type from
wire/tri to tri1 or tri0 depending on the pull. The issue is that
if the input net is not a wire or tri this is not safe and should
really be done as an external pull gate connected to the input.
We will need to handle this is it ever comes up. For now a sorry
message is printed.

Module.cc

@@ -30,6 +30,7 @@ Module::Module(perm_string n)
 {
       library_flag = false;
       is_cell = false;
+      uc_drive = UCD_NONE;
       default_nettype = NetNet::NONE;
       timescale_warn_done = false;
 }