This adds support for notifying systemd about the state of LND. It
notifies systemd just before waiting for wallet password or, if
`wallet-password-file` was specified, right after unlocking the wallet.
This means that "ready" represents RPC being available for intended use.
It's intentional, so that client services can use `After=` in `systemd`
configuration to avoid misleading error messages about missing files or
refused connections.
Part of #4470