Fix loop index var types, fixing many minor sign comparison warnings

foo.size() typically returns an unsigned integral type; make loop variables
match those types' signedness.

src/wallet.h

@@ -402,7 +402,7 @@ public:
     bool UpdateSpent(const std::vector<char>& vfNewSpent)
     {
         bool fReturn = false;
-        for (int i=0; i < vfNewSpent.size(); i++)
+        for (unsigned int i = 0; i < vfNewSpent.size(); i++)
         {
             if (i == vfSpent.size())
                 break;
@@ -488,7 +488,7 @@ public:
             return nAvailableCreditCached;
 
         int64 nCredit = 0;
-        for (int i = 0; i < vout.size(); i++)
+        for (unsigned int i = 0; i < vout.size(); i++)
         {
             if (!IsSpent(i))
             {
@@ -541,7 +541,7 @@ public:
         std::vector<const CMerkleTx*> vWorkQueue;
         vWorkQueue.reserve(vtxPrev.size()+1);
         vWorkQueue.push_back(this);
-        for (int i = 0; i < vWorkQueue.size(); i++)
+        for (unsigned int i = 0; i < vWorkQueue.size(); i++)
         {
             const CMerkleTx* ptx = vWorkQueue[i];