Removing new trade hotkeys while a counteroffer is displayed.

Fixes http://www.bay12forums.com/smf/index.php?topic=126076.1140
It's tempting to use has_offer for this effect, but that flag remains true after hitting Enter to consider the offer.
Relies on a structure update: https://github.com/DFHack/df-structures/pull/18

plugins/autotrade.cpp

@@ -481,6 +481,13 @@ struct tradeview_hook : public df::viewscreen_tradegoodsst
     {
         INTERPOSE_NEXT(render)();
         
+        if (counteroffer.size() > 0)
+        {
+            // The merchant is proposing a counteroffer,
+            // so there is nothing to mark.
+            return;
+        }
+        
         // Insert into the blank line between trade items and standard keys.
         // The blank line at the bottom is taken by the search plugin.
         auto dim = Screen::getWindowSize();

plugins/search.cpp

@@ -1074,6 +1074,14 @@ class trade_search_merc : public trade_search_base
 public:
     virtual void render() const
     {
+        if (viewscreen->counteroffer.size() > 0)
+        {
+            // The merchant is proposing a counteroffer.
+            // Not only is there nothing to search,
+            // but the native hotkeys are where we normally write.
+            return;
+        }
+        
         print_search_option(2, -1);
 
         if (!search_string.empty())
@@ -1116,6 +1124,14 @@ class trade_search_fort : public trade_search_base
 public:
     virtual void render() const
     {
+        if (viewscreen->counteroffer.size() > 0)
+        {
+            // The merchant is proposing a counteroffer.
+            // Not only is there nothing to search,
+            // but the native hotkeys are where we normally write.
+            return;
+        }
+        
         print_search_option(42, -1);
 
         if (!search_string.empty())