Hello,

I am attaching an updated patch for ox-icalendar unscheduled and
repeating TODOs, incorporating some of Ihor's feedback to my RFC some
months ago.

Compared to my original RFC, here are the main changes:

- For unscheduled TODOs with repeating deadline, the deadline warning
  days is used as the start time by default, in order to comply with
  the iCalendar spec which demands a start time in this case.

- Previously I had separate patches for unscheduled and repeating
  TODOs, but now I combine them into a single patch because of the way
  repeats and start times are intertwined for repeating deadlines.

- New customization `org-icalendar-todo-unscheduled-start' controls
  the exported start time for unscheduled TODOs. It replaces
  `org-icalendar-todo-force-scheduling' from my previous version of
  the patch.

- In case of a SCHEDULED repeater, and a DEADLINE with no repeater,
  the task repeats until the deadline, using the RRULE UNTIL keyword.

- Added linting for the case where SCHEDULED and DEADLINE have
  mismatching repeaters.

- Added several tests for ox-icalendar, and a test for the new lint as well.

There are still a few cases that are not yet handled, but they are
less common and will take some more work to implement, so I would
prefer to leave them to future patches:

- Case where SCHEDULED and DEADLINE have mismatched repeaters.  We can
  use RDATE with differing DURATION for this.

- Case where DEADLINE has repeater but SCHEDULED does not.  We can use
  RDATE for the first instance, and RRULE for the subsequent repeats.

- Case of catch-up "++" repeaters.  We can use EXDATE to exclude
  repeats before today.

- Case of restart ".+" repeaters.  I don't think iCalendar can handle
  this case, and we should ignore it.