Handle corrupt wallets gracefully.

Corrupt wallets used to cause a DB_RUNRECOVERY uncaught exception and a
crash. This commit does three things:

1) Runs a BDB verify early in the startup process, and if there is a
low-level problem with the database:
  + Moves the bad wallet.dat to wallet.timestamp.bak
  + Runs a 'salvage' operation to get key/value pairs, and
    writes them to a new wallet.dat
  + Continues with startup.

2) Much more tolerant of serialization errors. All errors in deserialization
are reported by tolerated EXCEPT for errors related to reading keypairs
or master key records-- those are reported and then shut down, so the user
can get help (or recover from a backup).

3) Adds a new -salvagewallet option, which:
 + Moves the wallet.dat to wallet.timestamp.bak
 + extracts ONLY keypairs and master keys into a new wallet.dat
 + soft-sets -rescan, to recreate transaction history

This was tested by randomly corrupting testnet wallets using a little
python script I wrote (https://gist.github.com/3812689)

src/wallet.h

@@ -16,11 +16,11 @@
 #include "script.h"
 #include "ui_interface.h"
 #include "util.h"
+#include "walletdb.h"
 
 class CAccountingEntry;
 class CWalletTx;
 class CReserveKey;
-class CWalletDB;
 class COutput;
 
 /** (client) version numbers for particular wallet features */
@@ -256,7 +256,7 @@ public:
     }
     void SetBestChain(const CBlockLocator& loc);
 
-    int LoadWallet(bool& fFirstRunRet);
+    DBErrors LoadWallet(bool& fFirstRunRet);
 
     bool SetAddressBookName(const CTxDestination& address, const std::string& strName);