Display yearly ical events from first year on. Fix Bug#23100.

Display yearly ical events from first year on.  Fix Bug#23100.

Convert yearly rrule starting in year x into diary-anniversary entry
for year x-1 when importing an icalendar.  Correspondingly convert
diary-anniversary for year x into yearly rrule starting in year x+1.

*
test/lisp/calendar/icalendar-resources/import-rrule-anniversary.diary-american:
*
test/lisp/calendar/icalendar-resources/import-rrule-anniversary.diary-european:
*
test/lisp/calendar/icalendar-resources/import-rrule-anniversary.diary-iso:
*
test/lisp/calendar/icalendar-resources/import-rrule-yearly.diary-american:
*
test/lisp/calendar/icalendar-resources/import-rrule-yearly.diary-european:
*
test/lisp/calendar/icalendar-resources/import-rrule-yearly.diary-iso:
*
test/lisp/calendar/icalendar-tests.el (icalendar-convert-anniversary-to-ical):
Match new diary-anniversary/yearly-rrule behaviour.

* lisp/calendar/icalendar.el (icalendar--datestring-to-isodate): Add
year-shift option.  (icalendar--convert-anniversary-to-ical): Shift
the year as diary-anniversary is not displayed in the initial year.
(icalendar--convert-recurring-to-diary): Shift the year as
diary-anniversary is not displayed in the initial year. (Bug#23100)