time-util: extract parse_calendar_date() from sysupdate

Move the YYYY-MM-DD date parsing and validation logic from
sysupdate-resource.c into a shared parse_calendar_date() function
in time-util, so it can be reused by other subsystems.

src/basic/time-util.h

@@ -180,6 +180,7 @@ const char* etc_localtime(void);
 
 int mktime_or_timegm_usec(struct tm *tm, bool utc, usec_t *ret);
 int localtime_or_gmtime_usec(usec_t t, bool utc, struct tm *ret);
+int parse_calendar_date(const char *s, usec_t *ret);
 
 uint32_t usec_to_jiffies(usec_t usec);
 usec_t jiffies_to_usec(uint32_t jiffies);