Use a new function to mark anniversaries in Org diary files

Use a new function to mark anniversaries in Org diary files

* lisp/org-agenda.el (org-agenda-add-entry-to-org-agenda-diary-file):
Use stable internal `org-anniversary' instead of diary-anniversary.
(org-class): New function.
(org-diary-class): Use `org-class'.
(org-anniversary, org-cyclic, org-date, org-block): New functions.

This patch provides stable alternatives for a number of diary
functions to be used in diary sexp entries. The corresponding diary-*
functions swap around their input arguments depending on
`calendar-date-style', which is unstable and evil.  The functions
provided here have a fixed order of arguments, the ISO order: year
month day.

Also, the `i a' key in the agenda now uses `org-anniversary' instead of
diary-anniversary.