This modifies the `genMacaroons` logic to indepently check for each of
the three default macaroons (admin, readonly, invoice) and generate
whichever are missing. Previously, this was an all or nothing routine.
In other words, either all three didn't exist on disk and all three are
created, or no macaroons are created. Although that works for the first
run of a new node, it can result in inconsistent states if only one or
two of the macaroons is deleted.
See https://github.com/lightningnetwork/lnd/discussions/7566.
16463d4bd4