Don't use sets when calculating the modlist so order is preserved.
    
    This is for the LDAP updater in particular. When adding new schema
    order can be important when one objectclass depends on another via
    SUP.
    
    This calculation will preserve the order of changes in the update file.
    
    Discovered trying to add SSH schema.
    
    https://fedorahosted.org/freeipa/ticket/754