New file 'pre1970' for zones that differ only in pre-1970 time stamps.

This lets us preserve information about pre-1970 time stamps when
we change a Zone to a Link to another zone whose time stamps agree
after 1970.  This should address concerns about some recent
changes that removed this information.  This implementation is a
stripped-down version of a suggestion by Andrew Main (Zefram) in
<http://mm.icann.org/pipermail/tz/2013-August/019615.html> and
<http://mm.icann.org/pipermail/tz/2013-August/019639.html> to
allow filtering tz data by date range.  Unlike Zefram's
suggestion, this implementation supports only two date ranges,
namely 1970 on, using 'make BACKWARD=backward'; and all dates,
using 'make BACKWARD="pre1970 back-pre1970"'.  At some point I'd
like to improve it to support arbitrary date ranges, but at least
we've now restored the data whose loss was of some concern.
* .gitignore: Add back-pre1970.
* Makefile (BACKWARD): New macro.
(YDATA): Use it instead of 'backward'.
(AWK_SCRIPTS): New macro, with additional script back-pre1970.awk.
(MISC): Use it.
(back-pre1970): New rule.
(clean_misc): Clean back-pre1970.  Also clean time.tab, while we're
at it.
(check_public): Don't require pre1970 to stand alone.
* pre1970, back-pre1970.awk: New files.