reintegrate NetBSD's false builtin

bin/sh/builtins

@@ -64,6 +64,7 @@ exitcmd		exit
 expcmd		exp let
 exportcmd	export readonly
 #exprcmd	expr test [
+falsecmd	false
 histcmd		fc
 fgcmd -j	fg
 getoptscmd	getopts

bin/sh/builtins.def

@@ -64,6 +64,7 @@ exitcmd		exit
 expcmd		exp let
 exportcmd	export readonly
 #exprcmd	expr test [
+falsecmd	false
 histcmd		fc
 fgcmd -j	fg
 getoptscmd	getopts

bin/sh/eval.c

@@ -916,6 +916,11 @@ returncmd(argc, argv)  char **argv; {
 }
 
 
+falsecmd(argc, argv)  char **argv; {
+	return 1;
+}
+
+
 truecmd(argc, argv)  char **argv; {
 	return 0;
 }