Update import of basic recurrence rules

Conversion of COUNT to UNTIL was a simple calculation which assumed one
repetiton per period (day, week, month or year); it does not take exception
days and invalid dates into account. Solved by a new function which returns the
n'th occurrence of a recurrence rule.

In calcurse UNTIL is interpreted as a day (DATE), in RFC 5545 as a time of day
(DATE-TIME). This has implications when a recurrence rule has an occurrence on
the UNTIL day, see comment in ical.c

An "Import:" note is added when a multi-day event is imported and turned into a
calcurse all-day event.

Icalendar quotes in comments have been updated to RFC 5545.

Signed-off-by: Lars Henriksen <LarsHenriksen@get2net.dk>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

test/data/ical-009.ical

@@ -111,6 +111,17 @@ RRULE:FREQ=MONTHLY;UNTIL=20201030
 EXDATE;VALUE=DATE:20200606T120000Z
 SUMMARY:Invalid EXDATE value
 END:VEVENT
+BEGIN:VEVENT
+DTSTART:20200527T163000
+DTEND:20200528T163000
+DURATION:P1D
+SUMMARY:Both end time and duration
+END:VEVENT
+BEGIN:VEVENT
+DTSTART:20200527T163000
+DTEND:20200526T163000
+SUMMARY:End time before start
+END:VEVENT
 BEGIN:VTODO
 SUMMARY:finally\, missing end of item
 END:VCALENDAR