KRB5: Move checking for illegal RE to krb5_utils.c
    
    Otherwise we would have to link krb5_child with pcre and transfer the
    regex, which would be cumbersome. Check for illegal patterns when
    expanding the template instead.
    
    Related:
    https://fedorahosted.org/sssd/ticket/2370
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>