misc/zoneinfo: split BACKWARD into two options for recent/ancient backward compatibility

[andrew_tao173.riddles.org.uk]

Summary

This patch splits the BACKWARD option into separate BACKWARD and BACKWARD20 options, the first of which includes only changes since 2000a (which respect the current naming scheme) while the second includes all of the old garbage as well.

Make BACKWARD the default.

Note that matching the base system's recent behavior change (see D24306) would require making BACKWARD20 the default instead, but I think this is a better solution.

2000 as a cutoff date was chosen because (a) convenient round number and (b) it excludes all the old misnamed zones like GB-Eire.

(Note that this depends on updating to 2020a first)

[philip]

I have no objection to this patch but I worry about FreeBSD diverging more from other operating systems. People have expectations about backwards compatible timezone lines existing. Breaking software gratuitously because we have a subset of compat links (even if the compat links by themselves are long stale) would not be cool.

I think a discussion about a split between backward and backward2000 should take place on the [e-mail] mailing list rather than in a review for a FreeBSD ports patch.

Could you please start a discussion about this upstream?

[andrew_tao173.riddles.org.uk]

Discussion opened upstream at http://mm.icann.org/pipermail/tz/2020-May/028970.html

[philip]

Thank you!