LDAP: Adding SIGCHLD callback
    
    This patch adds SIGCHLD callback for ldap_child. So if timeout is
    reached and ldap_child is terminated by handler we have debug message
    about it.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3106
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>