SYSDB: Allow storing non-POSIX users
    
    Related to:
    https://pagure.io/SSSD/sssd/issue/3310
    
    We already do the same for groups. If the user does not have UID number
    set but does have the POSIX: false attribute set, then we save the user
    with zero UID and the non-POSIX flag.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>