New data file 'backzone' for out-of-scope and/or poorly-sourced data.

* backzone: New file.
* Makefile (DATA): Add backzone.
(check_character_set): Check it.
(posix_packrat): New rule.
(.PHONY): Add it.
* NEWS: Document the above.
Move out-of-scope UTC+1 African zones to 'backzone'.

This illustrates how 'backzone' can be used to migrate
out-of-scope data out of the database proper.
* africa (Africa/Luanda, Africa/Porto-Novo, Africa/Douala, Africa/Bangui)
(Africa/Kinshasa, Africa/Brazzaville, Africa/Malabo, Africa/Libreville)
(Africa/Niamey): Change these from zones to links (to Africa/Lagos).
Move their old data from here ...
* backzone: ... to here.
* checktab.awk (tztab): Remove special cases for these zones.
* NEWS: Document the above.