internal/caravan/common.lua use trader over broker

When doing trading (i.e. the DF Trade window is open showing the two
columns), it *can* happen that you *have* a broker, but you actually
*trade* using a different unit.  This can be triggered by opening
the depot building view and choosing `Anyone requested at trade`,
repeating this until some unit that is not the broker shows up to
do the trading.
When this happens, the DFHack `Select trade goods` overlay shows
different obfuscated values than the DF Trade window.
This patch fixes that case by using the trader's appraisal skill if
trading is active.

Author: SilasD

internal/caravan/common.lua

@@ -36,6 +36,13 @@ end
 
 function get_broker_skill()
     local broker = dfhack.units.getUnitByNobleRole('broker')
+    local interface_trade = df.global.game.main_interface.trade
+    if interface_trade.open == true
+        and interface_trade.choosing_merchant == false
+        and interface_trade.fortress_trader ~= nil
+    then
+        broker = interface_trade.fortress_trader
+    end
     if not broker then return 0 end
     for _,skill in ipairs(broker.status.current_soul.skills) do
         if skill.id == df.job_skill.APPRAISAL then