Add UPnP lease duration advanced option

This is a new setting added that is pending addition to
libtorrent 1.2.6. This setting is important because some routers do
not support expiration times on port-maps and do not return an error
either, silently failing. The previous default of permanent leases is
retained.

src/base/bittorrent/session.h

@@ -367,6 +367,8 @@ namespace BitTorrent
         void setOutgoingPortsMin(int min);
         int outgoingPortsMax() const;
         void setOutgoingPortsMax(int max);
+        int UPnPLeaseDuration() const;
+        void setUPnPLeaseDuration(int duration);
         bool ignoreLimitsOnLAN() const;
         void setIgnoreLimitsOnLAN(bool ignore);
         bool includeOverheadInLimits() const;
@@ -647,6 +649,7 @@ namespace BitTorrent
         CachedSettingValue<int> m_slowTorrentsInactivityTimer;
         CachedSettingValue<int> m_outgoingPortsMin;
         CachedSettingValue<int> m_outgoingPortsMax;
+        CachedSettingValue<int> m_UPnPLeaseDuration;
         CachedSettingValue<bool> m_ignoreLimitsOnLAN;
         CachedSettingValue<bool> m_includeOverheadInLimits;
         CachedSettingValue<QString> m_announceIP;