[tz] Terminology mismatch in tzcode file format manpage / header

[Andrew Paprocki]

The file format is described with these fields:

.I tzh_ttisgmtcnt
The number of UTC/local indicators stored in the file.

.I tzh_ttisgmtcnt
UTC/local indicators, each stored as a one-byte value;
they tell whether the transition times associated with local time types
were specified as UTC or local time,
and are used when a time zone file is used in handling POSIX-style
time zone environment variables.

A number of variables in localtime.c, tzfile.h, and zic.c dealing with
these fields have variations of this "isgmt" substring.

This presents a naming conflict because technically GMT and UTC are
two different things. I find the inconsistent naming creeping into
functions which deal with the file format. I would like to prevent
variations of the string "GMT" from winding up in API names since the
definition of the field refers to UTC.

I propose wholesale replacing of the "isgmt" substring in tzcode files
with "isutc" so that the file format fields are named consistently
with their definition. If absolutely necessary, a #define can be used
in tzfile.h so that code referencing tzh_ttisgmtcnt will still
compile.

-Andrew