Prefer 'unsigned int' for loop index variables tested against ::size()

C++ STL ::size() generally returns unsigned, which implies that "int idx"
style of loop variable will generate a signed-vs-unsigned comparison warning
when testing the loop exit condition "idx < blah.size()"

Update areas of the bitcoin code where loop variables may be more properly and
correctly defined as unsigned.

src/wallet.cpp

@@ -1391,8 +1391,8 @@ bool CWallet::TopUpKeyPool()
         CWalletDB walletdb(strWalletFile);
 
         // Top up key pool
-        int64 nTargetSize = max(GetArg("-keypool", 100), (int64)0);
-        while (setKeyPool.size() < nTargetSize+1)
+        unsigned int nTargetSize = max(GetArg("-keypool", 100), 0LL);
+        while (setKeyPool.size() < (nTargetSize + 1))
         {
             int64 nEnd = 1;
             if (!setKeyPool.empty())