Implement shell builtins: cd, exec, umask (issue #15)

Add builtin dispatch in lcmd.c that checks __builtins table before
fork(), so cd/exec/umask operate on the shell process itself.

lbuiltin.h

@@ -0,0 +1,14 @@
+/*
+** $Id: lbuiltin.h $
+** Shell builtins (cd, exec, umask)
+** See Copyright Notice in lua.h
+*/
+
+#ifndef lbuiltin_h
+#define lbuiltin_h
+
+#include "lua.h"
+
+void luaopen_builtins(lua_State *L);
+
+#endif