UTIL: Add sss_create_dir()
    
    The newly added function helps us to create a new dir avoiding a
    possible TUCTOU issue.
    
    It's going to be used by the new session provider code.
    
    A simple test for this new function has also been provided.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/2995
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>