Make lsn_reset ("detach databases") optional and off by default.

Add an option -detachdb (and entry in OptionDialog), without which no
lsn_reset is called on addr.dat and blkindex.dat. That means these
files cannot be moved to a new environment, but shutdown can be
significantly faster. The wallet file is always lsn_reset'ed.

-detachdb corresponds to the old behaviour, though it is off by
default now to speed up shutdowns.

src/qt/optionsmodel.cpp

@@ -28,6 +28,8 @@ void OptionsModel::Init()
         SoftSetBoolArg("-upnp", settings.value("fUseUPnP").toBool());
     if (settings.contains("addrProxy") && settings.value("fUseProxy").toBool())
         SoftSetArg("-proxy", settings.value("addrProxy").toString().toStdString());
+    if (settings.contains("detachDB"))
+        SoftSetBoolArg("-detachdb", settings.value("detachDB").toBool());
 }
 
 bool OptionsModel::Upgrade()
@@ -121,6 +123,8 @@ QVariant OptionsModel::data(const QModelIndex & index, int role) const
             return QVariant(nDisplayUnit);
         case DisplayAddresses:
             return QVariant(bDisplayAddresses);
+        case DetachDatabases:
+            return QVariant(fDetachDB);
         default:
             return QVariant();
         }
@@ -204,6 +208,11 @@ bool OptionsModel::setData(const QModelIndex & index, const QVariant & value, in
             settings.setValue("bDisplayAddresses", bDisplayAddresses);
             }
             break;
+        case DetachDatabases: {
+            fDetachDB = value.toBool();
+            settings.setValue("detachDB", fDetachDB);
+            }
+            break;
         default:
             break;
         }