lnd+signal: implement systemd notify

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

lnd.go

@@ -537,6 +537,10 @@ func Main(cfg *Config, lisCfg ListenerCfg, interceptor signal.Interceptor) error
 	// to the default behavior of waiting for the wallet creation/unlocking
 	// over RPC.
 	default:
+		if err := interceptor.Notifier.NotifyReady(false); err != nil {
+			return err
+		}
+
 		params, err := waitForWalletPassword(
 			cfg, pwService, []btcwallet.LoaderOption{dbs.walletDB},
 			interceptor.ShutdownChannel(),
@@ -893,6 +897,10 @@ func Main(cfg *Config, lisCfg ListenerCfg, interceptor signal.Interceptor) error
 	// We transition the RPC state to Active, as the RPC server is up.
 	interceptorChain.SetRPCActive()
 
+	if err := interceptor.Notifier.NotifyReady(true); err != nil {
+		return err
+	}
+
 	// If we're not in regtest or simnet mode, We'll wait until we're fully
 	// synced to continue the start up of the remainder of the daemon. This
 	// ensures that we don't accept any possibly invalid state transitions, or