LDAP: Adding support for SIGTERM signal
    
    We add support for handling SIGTERM signal. If ldap_child receives
    SIGTERM signal it removes temporary file.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3106
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>