script: Remove magic numbers

This adds two new constants, MAX_OPS_PER_SCRIPT and
MAX_PUBKEYS_PER_MULTISIG.

src/script/script.cpp

@@ -170,7 +170,7 @@ unsigned int CScript::GetSigOpCount(bool fAccurate) const
             if (fAccurate && lastOpcode >= OP_1 && lastOpcode <= OP_16)
                 n += DecodeOP_N(lastOpcode);
             else
-                n += 20;
+                n += MAX_PUBKEYS_PER_MULTISIG;
         }
         lastOpcode = opcode;
     }