UTIL: exit() the forked process if exec()-ing a child process fails
When exec() fails, we should not attempt to continue, but just kill the
forked process. The patch adds this logic to the exec_child() and
exec_child_ex() functions to avoid code duplication
Resolves:
https://fedorahosted.org/sssd/ticket/3016
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>