wallet: warn against accidental unsafe older() import

BIP 379 allows height and time locks that have no consensus meaning in BIP 68 / BIP 112.
This is used by some protocols like Lightning to encode extra data, but is unsafe when
used unintentionally. E.g. older(65536) is equivalent to older(1).

This commit emits a warning when importing such a descriptor.

It introduces a helper ForEachNode to traverse all miniscript nodes.

src/wallet/rpc/backup.cpp

@@ -240,6 +240,10 @@ static UniValue ProcessDescriptorImport(CWallet& wallet, const UniValue& data, c
             }
             parsed_desc->ExpandPrivate(0, keys, expand_keys);
 
+            for (const auto& w : parsed_desc->Warnings()) {
+               warnings.push_back(w);
+            }
+
             // Check if all private keys are provided
             bool have_all_privkeys = !expand_keys.keys.empty();
             for (const auto& entry : expand_keys.origins) {