calendarspec: fix calculation of timespec iterations that fall onto a DST change

calendarspec: fix calculation of timespec iterations that fall onto a DST change

If we tested a candidate time that would fall onto the DST change, and we
realized that it is now a valid time ('cause the given "hour" is missing),
we would jump to to beginning of the next bigger time period, i.e. the next
day.

mktime_or_timegm() already tells us what the next valid time is, so let's reuse
this, and continue the calculations at this point. This should allow us to
correctly jump over DST changes, but also leap seconds and similar.  It should
be OK even multiple days were removed from calendar, similarly to the
Gregorian-Julian transition. By reusing the information from normalization, we
don't have to make assumptions what the next valid time is.

Fixes #13745.

$ TZ=Australia/Sydney faketime '2019-10-06 01:50' build/systemd-analyze calendar 0/1:0/1 --iterations 20 | grep Iter
       Iter. #2: Sun 2019-10-06 01:52:00 AEST
       Iter. #3: Sun 2019-10-06 01:53:00 AEST
       Iter. #4: Sun 2019-10-06 01:54:00 AEST
       Iter. #5: Sun 2019-10-06 01:55:00 AEST
       Iter. #6: Sun 2019-10-06 01:56:00 AEST
       Iter. #7: Sun 2019-10-06 01:57:00 AEST
       Iter. #8: Sun 2019-10-06 01:58:00 AEST
       Iter. #9: Sun 2019-10-06 01:59:00 AEST
      Iter. #10: Sun 2019-10-06 03:00:00 AEDT
      Iter. #11: Sun 2019-10-06 03:01:00 AEDT
      Iter. #12: Sun 2019-10-06 03:02:00 AEDT
      Iter. #13: Sun 2019-10-06 03:03:00 AEDT
      Iter. #14: Sun 2019-10-06 03:04:00 AEDT
      Iter. #15: Sun 2019-10-06 03:05:00 AEDT
      Iter. #16: Sun 2019-10-06 03:06:00 AEDT
      Iter. #17: Sun 2019-10-06 03:07:00 AEDT
      Iter. #18: Sun 2019-10-06 03:08:00 AEDT
      Iter. #19: Sun 2019-10-06 03:09:00 AEDT
      Iter. #20: Sun 2019-10-06 03:10:00 AEDT

$ TZ=Australia/Sydney faketime 2019-10-06 build/systemd-analyze calendar 2/4:30 --iterations=3
  Original form: 2/4:30
Normalized form: *-*-* 02/4:30:00
    Next elapse: Sun 2019-10-06 06:30:00 AEDT
       (in UTC): Sat 2019-10-05 19:30:00 UTC
       From now: 5h 29min left
       Iter. #2: Sun 2019-10-06 10:30:00 AEDT
       (in UTC): Sat 2019-10-05 23:30:00 UTC
       From now: 9h left
       Iter. #3: Sun 2019-10-06 14:30:00 AEDT
       (in UTC): Sun 2019-10-06 03:30:00 UTC
       From now: 13h left