date - Upgrade calculations, add some gnu date -d support

* Use long instead of int for numerous calculations, fixing a number of
date calculation overflow issues.

* Add -d support (a gnu date op

date - Upgrade calculations, add some gnu date -d support

* Use long instead of int for numerous calculations, fixing a number of
date calculation overflow issues.

* Add -d support (a gnu date option). Make -d an alias for -v.

In addition to the formats already supported, we now support
+/-Ndays seconds, minutes, hours, months, years, and abbreviations),
next ...,
last ...,
several month, day, and year arrangements,
h:m[:s] specifications,
and a few other things.

Not all of these can be combined with prefixes and there is still a lot
of missing support for gnu date compatibility. In particular the
handling of TZ= timezone specifications is mis-applied and support for
the "Z" suffix is missing.

Localization is still not properly supported for weekday and month names.

* Appears to fix easy-rsa (part of the openvpn package), though easy-rsa
still has an unrelated openssl/libressl issue.

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date(1): Add '-I' flag to output in ISO 8601 format

The '-I' flag accepts arguments of 'date', 'hours', 'minutes', and
'seconds', and makes date(1) output the date in ISO 8601 format.

This change i

date(1): Add '-I' flag to output in ISO 8601 format

The '-I' flag accepts arguments of 'date', 'hours', 'minutes', and
'seconds', and makes date(1) output the date in ISO 8601 format.

This change is derived from FreeBSD (revision 337332).

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