KRB5_CHILD: More restrictive umask
We could use more restrictive umask in krb5_child. I found out that
there is directory creation, but it is done by create_ccache_dir()
which has its own umask setup.
Resolves:
https://fedorahosted.org/sssd/ticket/2424
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>