Properly mirror child exit status in ksu

ksu attempts to exit with the same status as its child process, but
does not do so correctly.  Use WEXITSTATUS() to extract the exit code.
Reported by Todd Lubin.

https://github.com/krb5/krb5/commit/03c7df6fc683c68975db46c7afaa24f1a9a05fae
Author: Greg Hudson <ghudson@mit.edu>
Commit: 03c7df6fc683c68975db46c7afaa24f1a9a05fae
Branch: master
 src/clients/ksu/main.c   | 2 +-
 src/clients/ksu/t_ksu.py | 8 +++++---
 2 files changed, 6 insertions(+), 4 deletions(-)