org-agenda: Refactor org-agenda-overriding-header code
* lisp/org-agenda.el (org-agenda--insert-overriding-header): Add macro.
(org-agenda-list)
(org-search-view)
(org-todo-list)
(org-tags-view): Use macro.
(org-agenda-overriding-header): Update docstring.
* etc/ORG-NEWS: Explain that header can be disabled with empty string.
Replace org-agenda-overriding-header tests in these four functions with
calls to a macro, eliminating the duplicate code. Also, disable the
header when the variable is set to the empty string.