tzfile.5: Add information on version 2 format timezone files

Updated using information from the tzcode 2010l release at
ftp://elsie.nci.nih.gov/pub.
(It's an open question whether or not a version of tzfile.5
should live independently in man-pages. It was added to the
man-pages set many years ago. For now, I'll follow a
conservative course that causes least pain to downstream,
by continuing to maintain a separate copy in man-pages.)

See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=594219

Reported-by: Yitzchak Gale <gale@sefer.org>
Signed-off-by: Michael Kerrisk <mtk@konstanz.(none)>

man5/tzfile.5

@@ -1,7 +1,7 @@
 .\" @(#)tzfile.5	7.11
 .\" This file is in the public domain, so clarified as of
 .\" 1996-06-05 by Arthur David Olson <arthur_david_olson@nih.gov>.
-.TH TZFILE 5 1996-06-05 "" "Linux Programmer's Manual"
+.TH TZFILE 5 2010-08-31 "" "Linux Programmer's Manual"
 .SH NAME
 tzfile \- timezone information
 .SH SYNOPSIS
@@ -11,7 +11,9 @@ The timezone information files used by
 .BR tzset (3)
 begin with the magic characters "TZif" to identify then as
 timezone information files,
-followed by sixteen bytes reserved for future use,
+followed by a character identifying the version of the file's format
+(as of 2005, either an ASCII NUL ('\\0') or a '2')
+followed by fifteen bytes containing zeroes reserved for future use,
 followed by six four-byte values of type
 .IR long ,
 written in a "standard" byte order
@@ -57,7 +59,9 @@ each one tells which of the different types of "local time" types
 described in the file is associated with the same-indexed transition time.
 These values serve as indices into an array of
 .I ttinfo
-structures that appears next in the file;
+structures (with
+.I tzh_typecnt
+entries) that appear next in the file;
 these structures are defined as follows:
 .in +4n
 .sp
@@ -121,7 +125,7 @@ were specified as UTC or local time,
 and are used when a timezone file is used in handling POSIX-style
 timezone environment variables.
 .PP
-.I Localtime
+.BR localtime (3)
 uses the first standard-time
 .I ttinfo
 structure in the file
@@ -132,3 +136,15 @@ if either
 .I tzh_timecnt
 is zero or the time argument is less than the first transition time recorded
 in the file.
+.PP
+For version-2-format timezone files,
+the above header and data is followed by a second header and data,
+identical in format except that
+eight bytes are used for each transition time or leap-second time.
+After the second header and data comes a newline-enclosed,
+POSIX-TZ-environment-variable-style string for use in handling instants
+after the last transition time stored in the file
+(with nothing between the newlines if there is no POSIX representation for
+such instants).
+.SH SEE ALSO
+.BR ctime (3)