MFC r320702,r320703:

r320702:

Formalize LEAPSECONDS and OLDTIMEZONES in share/zoneinfo/... as
`MK_ZONEINFO_LEAPSECONDS_SUPPORT == yes` and
`MK_ZONEINFO_OLD_TIMEZONES_SUPPORT == yes`.

Keep `LEAPSECONDS` and `OLDTIMEZONES` for backwards compatibility,
but print out a warning notifying users that they should use the new
variables, in an effort to migrate them to the variables. This is being
done mostly for automated build tools, etc, that might rely on these
variables being set. The variables will be removed in the future on
^/head, e.g., after ^/stable/12 is cut.

Relnotes: yes

r320703:

Add tests to help verify Links functionality for .../contrib/tzdata/backwards

MFC with: r320702