policy: uncap datacarrier by default

Datacarrier output script sizes and output counts are now
uncapped by default.

To avoid introducing another startup argument, we modify the
OP_RETURN accounting to "budget" the spk sizes.

If a user has set a custom default, this results in that
budget being spent over the sum of all OP_RETURN outputs'
scripts in the transaction, no longer capping the number
of OP_RETURN outputs themselves. This should allow a
superset of current behavior while respecting the passed
argument in terms of total arbitrary data storage.

Co-authored-by: Anthony Towns <aj@erisian.com.au>

src/test/multisig_tests.cpp

@@ -144,7 +144,7 @@ BOOST_AUTO_TEST_CASE(multisig_IsStandard)
 
     const auto is_standard{[](const CScript& spk) {
         TxoutType type;
-        bool res{::IsStandard(spk, std::nullopt, type)};
+        bool res{::IsStandard(spk, type)};
         if (res) {
             BOOST_CHECK_EQUAL(type, TxoutType::MULTISIG);
         }