shared/calendarspec: abort calculation after 1000 iterations

We have a bug where we seem to enter an infinite loop when running in the
Europe/Dublin timezone. The timezone is "special" because it has negative SAVE
values. The handling of this should obviously be fixed, but let's use a
belt-and-suspenders approach, and gracefully fail if we fail to find an answer
within a specific number of attempts. The code in this function is rather
complex, and it's hard to rule out another bug in the future.