Restore previous behavior for inherited tags, now just more flexible

Restore previous behavior for inherited tags, now just more flexible

* org.el (org-get-tags-at): Remove duplicate inherited tags.

* org-agenda.el (org-agenda-show-inherited-tags): Allow to be
set to 'always or to a list of agenda types.
(org-agenda-finalize): Rework conditions under which inherited
tags should be made available even when not displayed.
(org-search-view, org-agenda-get-todos)
(org-agenda-get-timestamps, org-agenda-get-sexps)
(org-agenda-get-progress, org-agenda-get-scheduled)
(org-agenda-get-blocks): Use new possible values of
`org-agenda-show-inherited-tags'.

Thanks to Karl Voit and Tassilo Horn who first reported this.