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

go.mod

@@ -14,6 +14,7 @@ require (
 	github.com/btcsuite/btcwallet/wallet/txrules v1.0.0
 	github.com/btcsuite/btcwallet/walletdb v1.3.6-0.20210803004036-eebed51155ec
 	github.com/btcsuite/btcwallet/wtxmgr v1.3.1-0.20210803004036-eebed51155ec
+	github.com/coreos/go-systemd v0.0.0-20190321100706-95778dfbb74e
 	github.com/davecgh/go-spew v1.1.1
 	github.com/fsnotify/fsnotify v1.4.9 // indirect
 	github.com/go-errors/errors v1.0.1