config: add stricter config parsing to avoid dangerous RPC auth combos
config: add stricter config parsing to avoid dangerous RPC auth combos

In this commit, we add an additional constraint to the RPC
configuration parsing. Before this commit, it was possible to start lnd
with either RPC server listening on an external interface *without*
authentication disabled. After this commit, if a user tries to start
the RPC server listening on an external interface without any sort of
RPC authentication, then the daemon will fail to start up.

LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd