minor, complement idate/daysi date arithmetic by dateutils' yd.h for month-dom <-> year-doy conversions

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
fix, properly refactor daysies that are really just doys into daysi_sans_year()

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
fix, re-fix the leap year regression (toy7.1)

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'core/date-arith' into next

* core/date-arith:
  fix, re-fix the leap year regression (toy7.1)
  fix, properly refactor daysies that are really just doys into daysi_sans_year()
  minor, complement idate/daysi date arithmetic by dateutils' yd.h for month-dom <-> year-doy conversions