SYSDB: Only generate new UID in local domain
    
    To avoid issues where a user with no UID but without the posix=false
    flag was passed to sysdb, we only allow generating the new ID in the
    local domain. This might prevent bugs where non-POSIX users would get a
    UID created by sysdb which might allow accessing resources owned by that
    UID.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>