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/protocol.cpp

@@ -270,7 +270,7 @@ CInv::CInv(int typeIn, const uint256& hashIn)
 
 CInv::CInv(const std::string& strType, const uint256& hashIn)
 {
-    int i;
+    unsigned int i;
     for (i = 1; i < ARRAYLEN(ppszTypeName); i++)
     {
         if (strType == ppszTypeName[i])