multi: add migrate-wallet-to-watch-only flag

To enable converting an existing wallet with private key material into a
watch-only wallet on first startup with remote signing enabled, we add a
new flag. Since the conversion is a destructive process, this shouldn't
happen automatically just because remote signing is enabled.

sample-lnd.conf

@@ -1274,6 +1274,13 @@ litecoin.node=ltcd
 ; Valid time units are {s, m, h}.
 ; remotesigner.timeout=5s
 
+; If a wallet with private key material already exists, migrate it into a
+; watch-only wallet on first startup.
+; WARNING: This cannot be undone! Make sure you have backed up your seed before
+; you use this flag! All private keys will be purged from the wallet after first
+; unlock with this flag!
+; remotesigner.migrate-wallet-to-watch-only=true
+
 [gossip]
 
 ; Specify a set of pinned gossip syncers, which will always be actively syncing