Make 'timezone' and 'altzone' more compatible with 'tzname'.

* localtime.c (localsub): Don't bother updating 'tzname' again
when localtime is GMT, as tzset's value should be fine.  When
updating 'tzname', also update 'timezone' and 'altzone'
consistently, on platforms that define the latter two vars.
* NEWS: Document this.
Move two out-of-scope Indochina zones to 'backzone'.

* asia (Asia/Phnom_Penh, Asia/Vientiane):
Change these from zones to links to Asia/Bangkok.
* backzone: Move the old data here.
* checktab.awk (tztab): Remove special cases for these zones.
* NEWS: Document the above.
Many corrections to data for Vietnam, 1906-1975.

* asia (Asia/Ho_Chi_Minh):
Rewrite the data from 1906 through 1975, using new information
from Trần Tiến Bình's book.  (Thanks to Trần Ngọc Quân.)
New abbreviation IDT for UT+8 in Indochina.
* checktab.awk (tztab): Remove special cases for this zone.
* zone1970.tab: Add Asia/Ho_Chi_Minh, and add Asia/Bangkok commentary.
* NEWS: Document this.
* zdump.c (adjusted_yday): Define only if TM_GMTOFF is not defined.
Fix some localtime.c problems noted by Christos Zoulas

in: http://mm.icann.org/pipermail/tz/2014-October/021684.html
* localtime.c (tzload): Use ssize_t, not int.  Redo comparison to
avoid the need for an ssize_t value less than -1, which POSIX does
not guarantee.  Use memmove so that we needn't worry about an
ssize_t index.
(tzparse): Remove static always-zero var.  Initialized fields
to zero by hand instead.
(zoneinit): Initialized more fields, to avoid undefined behavior
in tzalloc.
(leapcorr): Fix bug, a stray initialization of a local variable.
* Makefile: Add comment about ssize_t.
* NEWS: Document the above.