fix inheritance of documentclass options

Inheritance for documentclass options is not correct.

See \documentclass[DIV=12]{ic-book}.

Proper order in cls needs to be:

1. \keys_define:nn {l3icbook} { ... DIV ... }
2. \DeclareUnknownKeyHandler
3. \ProcessKeysOptions{l3icbook} to use relevant options and set initials
4. \ProcessKeyOptions to handle remaining options and send them to scrbook.
5. Remove \PassOptionsToPackage for setting default

Like this:

\keys_define:nn {l3icbook} {
    DIV       .code:n     = {\PassOptionsToClass{DIV = #1}{\g_icbook__base_class_str}},
    DIV       .initial:n  = 11,
    DIV       .value_required:n = true
  }