Fix calculation of the recurring weekdays

Both the result of the first_dayofweek_of_year and the target
weekday are zero-based (0 fo sunday) while the target month-day
or year-day is 1-based. Adjust logic accordingly.

Also add testcase for this PR to the kyua test suite

PR: 201062
Submitted by: Richard Narron <comet.berkeley@gmail.com>
MFC after: 1 week