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/fuzz/script.cpp

@@ -53,7 +53,7 @@ FUZZ_TARGET(script, .init = initialize_script)
     }
 
     TxoutType which_type;
-    bool is_standard_ret = IsStandard(script, std::nullopt, which_type);
+    bool is_standard_ret = IsStandard(script, which_type);
     if (!is_standard_ret) {
         assert(which_type == TxoutType::NONSTANDARD ||
                which_type == TxoutType::NULL_DATA ||