Revert recent pre-1970 changes.

Other tz file readers had problems with the new pre1970 file, so
remove it and related changes.  To recover the pre-1970 data,
instead change pre-1970 Zone entries in other files back to what
they were.  Perhaps we'll try to find a better way someday.
* .gitignore: Remove back-pre1970.
* Makefile (BACKWARD): Remove.  All uses changed back to 'backward'.
(AWK_SCRIPTS): Remove back-pre1970.awk.
(back-pre1970): Remove.
(clean_misc): Don't rm back-pre1970.
(check_public): Remove special case for pre1970.
* back-pre1970.awk, pre1970: Remove.
* backward (America/Anguilla, America/Antigua, America/Aruba)
(America/Atikokan, America/Blanc-Sablon, America/Cayman)
(America/Creston, America/Curacao, America/Dominica, America/Grenada)
(America/Guadeloupe, America/Montreal, America/Montserrat)
(America/Nassau, America/Port_of_Spain, America/St_Kitts)
(America/St_Lucia, America/St_Thomas, America/St_Vincent)
(America/Tortola): Remove, as these are zones again.
(America/Coral_Harbour, America/Kralendijk, America/Lower_Princes)
(America/Marigot, America/St_Barthelemy, America/Virgin):
Revert to previous links.
* northamerica (Mont, America/Blanc-Sablon, America/Montreal)
(America/Atikokan, America/Creston, America/Anguilla, America/Antigua)
(Bahamas, America/Nassau, America/Cayman, America/Dominica)
(America/Grenada, America/Guadeloupe, America/Montserrat)
(America/St_Kitts, America/St_Lucia, America/St_Vincent)
(America/Tortola, America/St_Thomas):
* southamerica (America/Aruba, America/Curacao, America/Port_of_Spain):
Restore these rules and zones.

* .gitignore: Add back-pre1970.
* Theory: Describe LMT better.

Following a suggestion by Guy Harris in
<http://mm.icann.org/pipermail/tz/2013-August/019706.html>.