Add support for NLS document building.

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.

ja/Makefile

@@ -0,0 +1,6 @@
+#       From: @(#)Makefile      8.1 (Berkeley) 6/5/93
+#       $Id: Makefile,v 1.1 1996-12-18 07:14:37 asami Exp $
+
+SUBDIR= handbook
+
+.include <bsd.subdir.mk>

ja_JP.EUC/Makefile

@@ -0,0 +1,6 @@
+#       From: @(#)Makefile      8.1 (Berkeley) 6/5/93
+#       $Id: Makefile,v 1.1 1996-12-18 07:14:37 asami Exp $
+
+SUBDIR= handbook
+
+.include <bsd.subdir.mk>

ja_JP.eucJP/Makefile

@@ -0,0 +1,6 @@
+#       From: @(#)Makefile      8.1 (Berkeley) 6/5/93
+#       $Id: Makefile,v 1.1 1996-12-18 07:14:37 asami Exp $
+
+SUBDIR= handbook
+
+.include <bsd.subdir.mk>