* zic.c (stringzone): Add support for perpetual DST.

This causes zic to generate, e.g., TZ='WART4WARST,J1/0,J365/24'
for perpetual DST in San Luis, Argentina.
See <http://mm.icann.org/pipermail/tz/2013-September/020056.html>.
Improve the support for perpetual DST.

Problem reported by Zefram in
<http://mm.icann.org/pipermail/tz/2013-September/020059.html>.
* localtime.c (tzparse): Elide simultaneous entries out of and
into DST if DST goes for an hour more than a year (actually, for
more than the DST offset more than a year).  Since this
optimization can elide all entries, avoid looping forever looking
for entries that will never arrive.  While we're at it, fix
another portability bug where the code assumed wraparound on
signed integer overflow.
* newtzset.3, tzfile.5: Mention that as an extension to POSIX,
if DST covers the entire year plus the DST offset, it's assumed to
be in effect all year.  Give an example.
* zic.c (stringrule): Omit the "J" in January and February,
as this can save a byte or two in the output.
(rule_cmp): New function.
(stringzone): Do a better job of constructing the standard-time
abbreviation when there is perpetual DST.  Defer to the new
stringrule to construct the times for perpetual DST.
Fix bug noted by Zefram, which caused a stray hour of standard
time to be inserted in an otherwise perpetual DST.
Previously, this code generated "WARST4WARST,J1/0,J365/24"
for the San Luis example; now it generates "WART4WARST,0/0,J365/25".