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.

MFC after: 1 month
Relnotes: yes
Reviewed by: bdrewery
Differential Revision: D11376