Log a message when shutting down because STDIN is closed

[hacklschorsch]

#3921 introduced a new behavior for tahoe run where the process will immediately exit when stdin is closed.

#4036 introduced a way to still run tahoe even when we don't have a STDIN (for example when running under systemd).

I was indeed trying to run tahoe under systemd and it took me over a day to find the --allow-stdin-close option (introduced with ​PR 1306).

I propose we add logging to Tahoe-LAFS 'run' so that when it shuts down because STDIN is closed users can know this, something like:

STDIN (Standard Input) closed, shutting down.  Use `--allow-stdin-close` if you want to keep Tahoe-LAFS running when that happens.