policy: make anchor spend standard

Only standard when non-nested.

src/script/interpreter.cpp

@@ -1943,6 +1943,8 @@ static bool VerifyWitnessProgram(const CScriptWitness& witness, int witversion,
             }
             return set_success(serror);
         }
+    } else if (!is_p2sh && CScript::IsPayToAnchor(witversion, program)) {
+        return true;
     } else {
         if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM) {
             return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM);