misc/zoneinfo: install zone1970.tab and other fixes

Install zone1970.tab in addition to zone.tab (like D20646 for base).

The old logic to remove countries from zone.tab doesn't port to
zone1970.tab since some lines might have to be patched more than
once. Replace it with some awk.

Remove LEAPFILE reference that wasn't being set anyway.

Fix plist entry for Etc/UCT, which is (since 2019a) part of
"backward" and not present otherwise.

Make BACKWARD not be the default, since (a) it's not the base system's
default and (b) that stuff is all deprecated.

Update comment about check-plist to include a reminder to check both with
and without BACKWARD (this would have caught the existing plist error).

Remove no-longer-needed include of bsd.port.pre.mk.

Fix location and name of iso3166 file in comments in zone1970.tab, also
to match base system.

Add back a test-pkgplist target, because it turns out check-plist is not
strict enough to handle the case of moving entries into or out of the
"backward" set; check-plist doesn't error if a file is present in STAGEDIR
but is commented out in the plist, but we want that to be noticed.

Submitted by: Andrew Gierth <andrew@tao11.riddles.org.uk>
Reviewed by: philip, skreuzer, tmunro
Approved by: skreuzer (maintainer), allanjude (src mentor)
Differential Revision: https://reviews.freebsd.org/D20981