Adapts the gui(s) to detect if an existing reserve input has not been
used as input for the transaction even though the user tried to spend
max. This allows to show the lightning reserve warning not only for
reserve change outputs but also for existing reserve inputs that have
been ignored for this max spend transaction.
Still keeps the `is_utxo_reserve` flag on `PartialTxOutput` as it is
used in the qml gui.
30b146033d