calendarspec: be more graceful with two kinds of calendar expressions

calendarspec: be more graceful with two kinds of calendar expressions

This changes the calendarspec parser to allow expressions such as
"00:05..05", i.e. a range where start and end is the same. It also
allows expressions such as "00:1-2/3", i.e. where the repetition value
does not fit even once in the specified range. With this patch both
cases will now be optimized away, i.e. the range is removed and a fixed
value is used, which is functionally equivalent.

See #15030 for an issue where the inability to parse such expressions
caused confusion.

I think it's probably better to accept these gracefully and optimizing
them away instead of refusing them with a plain EINVAL. With a tool such
as "systemd-analyze" calendar it should be easy to figure out the
normalized form with the redundant bits optimized away.