Add SCRIPT_VERIFY_MINIMALDATA (BIP62 rules 3 and 4)

Also use the new flag as a standard rule, and replace the IsCanonicalPush
standardness check with it (as it is more complete).

src/main.cpp

@@ -633,10 +633,6 @@ bool IsStandardTx(const CTransaction& tx, string& reason)
             reason = "scriptsig-not-pushonly";
             return false;
         }
-        if (!txin.scriptSig.HasCanonicalPushes()) {
-            reason = "scriptsig-non-canonical-push";
-            return false;
-        }
     }
 
     unsigned int nDataOut = 0;