SDAP: Relax POSIX check
    
    Relax the check on UID or GID just to check if at least one of them is
    present but do not require them to be positive numbers.
    
    Add requirement on objectclass attributes to be user or group to make
    check more reliable.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2800
    (cherry picked from commit 6735c0451d4e80d7cd4b480a8c1f7dafb2b536ea)
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>