app-shells/zsh: various improvements
app-shells/zsh: various improvements

- Recipe changes:
  - A SUMMARY that better reflects zsh
  - Added GLOBAL_WRITABLES_FILES and USER_SETTINGS_FILES
  - LICENSE and COPYRIGHT updated
  - Introduced the _doc package with HTML and PDF documentations
  - Global configuration files has been moved to the correct location
  - Various part has been cleaned up
  - A fake home directory is created for TEST() to keep user home
    directory clean, if we ever to support non-chroot building

- Functional changes:
  - zsh now sources /etc/profile by default
  - Unless the user has dotfiles in their home directory, place them
    in B_USER_SETTINGS_DIRECTORY/zsh by default
    - If the users rely on the old behavior that
      B_USER_SETTINGS_DIRECTORY/zsh/z* is sourced, appropriate symlinks
      will be generated.

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