simplewallet: add a warning and prompt on rescan_blockchain

Many people are using this as a "let's see what this does" command
when something doesn't work as they thought it should, and thus
destroying info that they might still need.

src/simplewallet/simplewallet.cpp

@@ -2201,7 +2201,7 @@ simple_wallet::simple_wallet()
                            tr("Show the unspent outputs of a specified address within an optional amount range."));
   m_cmd_binder.set_handler("rescan_bc",
                            boost::bind(&simple_wallet::rescan_blockchain, this, _1),
-                           tr("Rescan the blockchain from scratch."));
+                           tr("Rescan the blockchain from scratch, losing any information which can not be recovered from the blockchain itself."));
   m_cmd_binder.set_handler("set_tx_note",
                            boost::bind(&simple_wallet::set_tx_note, this, _1),
                            tr("set_tx_note <txid> [free text note]"),
@@ -6332,6 +6332,14 @@ bool simple_wallet::unspent_outputs(const std::vector<std::string> &args_)
 //----------------------------------------------------------------------------------------------------
 bool simple_wallet::rescan_blockchain(const std::vector<std::string> &args_)
 {
+  message_writer() << tr("Warning: this will lose any information which can not be recovered from the blockchain.");
+  message_writer() << tr("This includes destination addresses, tx secret keys, tx notes, etc");
+  std::string confirm = input_line(tr("Rescan anyway ? (Y/Yes/N/No): "));
+  if(!std::cin.eof())
+  {
+    if (!command_line::is_yes(confirm))
+      return true;
+  }
   return refresh_main(0, true);
 }
 //----------------------------------------------------------------------------------------------------