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.