* zic.c: Prefer <stdarg.h> to allocating memory by hand.

Since we assume C89 now, it's safe to use vfprintf.
(verror): New function, with the old implementation of 'error'
but with a va_list API.
(error, warning): Use it.
(associate, gethms): Pass explicit %s to avoid GCC warning about
possibly unsafe formats.
(inzone, outzone, newabbr): Simplify by using the new error or
warning functions, avoiding the need to allocate and free memory.
* private.h (ATTRIBUTE_FORMAT): New macro.
Bump tzfile format to version 3.

Also, improve the documentation and diagnostics in this area.
Suggested by Arthur David Olson in
<http://mm.icann.org/pipermail/tz/2013-September/020064.html>.
* tzfile.5, tzfile.h: Bump tzfile format to version 3.
* zic.8: Document -v better.
* zic.c (ZIC_VERSION): Bump from '2' to '3'.
(stringrule, stringzone, outzone): Report compatibility issues
more carefully, mentioning client dates.
* zic.c: Tweak 400-years-hack to better handle some edge cases better.

Derived from Zefram's patch mentioned in
<http://mm.icann.org/pipermail/tz/2013-July/019470.html>.
With the current tz data, this affects only the Asia/Tehran file,
and it doesn't affect zdump output.
(YEAR_BY_YEAR_ZONE): New constant.
(stringzone): Return it when applicable.
(outzone): Search through a couple of extra years when extending.
Extend when we ran past 2037 in the data, too.