tty: handle misconfigured namespaces

tty: handle misconfigured namespaces

On some platforms, isatty succeeds but ttyname fails.
POSIX does not seem to allow this, but there it is.
Problem reported by Christian Brauner (Bug#26371).
While we’re at it, check for errors more carefully and return a
new exit status 4 if stdin is closed or a similar error occurs.
* doc/coreutils.texi (tty invocation): Document new behavior.
* init.cfg (stderr_fileno_):
Don't assume have_input_tty is not in the environment.
* src/tty.c (TTY_STDIN_ERROR): New constant.
(main): Exit with nonzero status if there is a usage error,
like other coreutils programs.
Check for error in getting stdin type.
* tests/misc/tty.sh: New file.
* tests/local.mk (all_tests): Add it.