analyze: add new "calendar" command

This little new command can parse, validate, normalize calendar events,
and calculate when they will elapse next. This should be useful for
anyone writing calendar events and who'd like to validate the expression
before running them as timer units.

src/basic/calendarspec.h

@@ -61,3 +61,5 @@ int calendar_spec_to_string(const CalendarSpec *spec, char **p);
 int calendar_spec_from_string(const char *p, CalendarSpec **spec);
 
 int calendar_spec_next_usec(const CalendarSpec *spec, usec_t usec, usec_t *next);
+
+DEFINE_TRIVIAL_CLEANUP_FUNC(CalendarSpec*, calendar_spec_free);