Code clean-up: rename some agenda internals.
* org.el (org-agenda-prepare-buffers): Rename from
`org-prepare-agenda-buffers'.
(org-match-sparse-tree, org-map-entries): Use the new names.
* org-agenda.el (org-agenda-prepare-window): Rename from
`org-prepare-agenda-window'.
(org-agenda-prepare): Rename from `org-prepare-agenda'.
(org-agenda-run-series, org-agenda-prepare, org-timeline)
(org-agenda-list, org-search-view, org-todo-list)
(org-tags-view, org-agenda-list-stuck-projects, org-diary)
(org-agenda-to-appt): Use the new names.
* org-mobile.el (org-mobile-create-index-file): Ditto.
* org-icalendar.el (org-export-icalendar): Ditto.
* org-clock.el (org-dblock-write:clocktable)
(org-dblock-write:clocktable): Ditto.
* org2rem.el (org2rem): Ditto.