su: fix exit status if terminated by signal

[kzak@redhat.com: - add comment]

Signed-off-by: Ondrej Oprala <ooprala@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>
mount: (deprecated) fix typo tailed->failed

Signed-off-by: Yuri Chornoivan <yurchor@ukr.net>