Move out-of-scope UTC+2 African zones to 'backzone'.

* africa (Africa/Blantyre, Africa/Bujumbura, Africa/Gaborone)
(Africa/Harare, Africa/Kigali, Africa/Lubumbashi, Africa/Lusaka):
Change these from zones to links to Africa/Maputo.
(Africa/Maseru, Africa/Mbabane):
Change these from zones to links to Africa/Johannesburg.
* backzone: Move the old data here.
* checktab.awk (tztab): Remove special cases for these zones.
* NEWS: Document the above.
Check that supposedly-sorted source files actually are sorted.

* Makefile (check_sorted): New rule.
(check, .PHONY): Add it.
* backzone: Sort.