calendar(1): Significant update to calendars from OpenBSD

* The calendars were obtained from OpenBSD as of 2019-02-19
* Generally, OpenBSD has more entries and more accurate, but FreeBSD has
also

calendar(1): Significant update to calendars from OpenBSD

* The calendars were obtained from OpenBSD as of 2019-02-19
* Generally, OpenBSD has more entries and more accurate, but FreeBSD has
also updated some entries and fixed some mistakes.
* I cross-checked most mismatching entries by searching the Internet and
also fixed several mistakes.
* NOTE: there can certainly exist quite some mistakes because I cannot
thoroughly check all of them.

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