[krbdev.mit.edu #4137] ksu spuriously fails when exiting shell when ksu-ing to non-root

[Tom Yu via RT]

Sachin Punadikar reports that ksu spuriously fails when exiting the
spawned shell when ksu-ing to a non-root user.  This is due to failure
from krb5_seteuid(0) after setuid(target_user).