Add driver for the HiZ of open nets.

Even open nets have a driver for HiZ. The BUFZ is needed as something
to hang .net objects. This is another step closer to not needing the
"create_constant_node" hack in vvp/words.cc.

tgt-vvp/draw_net_input.c

@@ -672,6 +672,16 @@ static void draw_net_input_x(ivl_nexus_t nex,
 		  }
 		  *tmp++ = '>';
 		  *tmp = 0;
+
+		    /* Create an "open" driver to hold the HiZ. We
+		       need to do this so that .nets have something to
+		       hang onto. */
+		  char buf[64];
+		  snprintf(buf, sizeof buf, "o%p", nex);
+		  fprintf(vvp_out, "%s .functor BUFZ %u, %s; HiZ drive\n",
+			  buf, wid, nex_private);
+		  nex_private = realloc(nex_private, strlen(buf)+1);
+		  strcpy(nex_private, buf);
 	    }
 
 	    if (island) {

vvp/words.cc

@@ -290,12 +290,17 @@ static void __compile_net(char*label,
 
       assert(argc == 1);
       vvp_net_t*node = vvp_net_lookup(argv[0].text);
+#if 1
       if (node == 0) {
 	      /* No existing net, but the string value may be a
 		 constant. In that case, we will wind up generating a
-		 bufz node that can carry the constant value. */
+		 bufz node that can carry the constant value.
+
+	         NOTE: This is a hack! The code generator should be
+	         fixed so that this is no longer needed. */
 	    node = create_constant_node(label, argv[0].text);
       }
+#endif
       if (node == 0) {
 	    struct __vpiScope*scope = vpip_peek_current_scope();
 	    __compile_net_resolv*res