Submitted by: Kazuo Horikawa <horikawa@isrd.hitachi.co.jp>

This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.

Boy, I'm glad we're not using sup anymore.  This update would have been
insane otherwise.

Here's the logic (mostly suggested by John Fieber):

 @ If ALLLANG is defined, descend to all language-specific subdirs too.
 @ If ALLLANG is not defined, but LANG is defined and a subdirectory with
   that name exists, descend to that directory too.
 @ In either case, the default subdirectories are always traversed.

Right now there's only one subdirectory (ja_JP.EUC) with one document
(handbook).  Note these changes won't do anything if you don't have ALLLANG
defined or LANG set to ja_JP.EUC.

The make world for releases is expected to define ALLLANG, so all language-
specific documentation can be built (and be put in appropriate *dists).

This should be in 2.2.