Fixes#5080.
The N_TTY_BUF_SIZE kernel parameter dictates how many characters can be
pasted into a terminal window. This cannot be circumvented by reading
the input in a different manner.
To avoid the problem fully, we instead allso allow the user to type in a
path to a text file that is read if it exists. That way the PSBT can be
as long as needed.
This commit adds a shutdown logger which will send a request for
shutdown on critical errors. It uses the signal package to request safe
shutdown of the daemon. Since we init our logs in config validation,
we add a started channel to the signal package to prevent the case where
we have a critical log after the ShutdownLogger has started but before
the daemon has started listening for intercepts. In this case, we just
ignore the shutdown request.
