chainrg: lnd uses wrong coin type for simnet and other networks

[Roasbeef]

Today lnd uses the wrong coin type for simnet and other networks: https://github.com/lightningnetwork/lnd/blob/master/chainreg/chainparams.go#L45-L58

We'll bail out of certain operations like PSBT signing if the coin type doesn't match what we expect. The cointype for testnet is 1, while the actual cointype for simnet is 115: https://github.com/btcsuite/btcd/blob/master/chaincfg/params.go#L758-L760

I think this was just a historical simplification, but we should likely update to start using th correct coin types, as I think maybe the "default" signet has a diff value. Alternatively, we can just mark this as wontfix as well.