never close stdin

never close stdin

The sanitise_stdfd call makes sure that standard file descriptors are
open (if they were closed, they are connected with /dev/null).

Do not close stdin in any case to prevent error messages when stdin is
read multiple times and to prevent later usage of fd 0 for connections,
e.g.

echo localhost | ssh-keyscan -f - -f -

While at it, make stdin-related error messages nicer.

Authored with Max Kunzelmann <maxdev at posteo dot de>

ok djm