login: prevent undefined ioctl and tcsetattr calls

login: prevent undefined ioctl and tcsetattr calls

Do not call tcsetattr if tcgetattr fails, because the content of
tt and ttt is undefined in that case.

Also do not just warn if ioctl fails, but also avoid calling it again
after tty has been re-opened.

I've solved this by setting struct variables to values which cannot be
valid at this point. If they do have these exact values, then the
calls will be prevented.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>