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.

linit.c

@@ -21,6 +21,7 @@
 #include "lauxlib.h"
 #include "llimits.h"
 #include "lcmd.h"
+#include "lbuiltin.h"
 
 
 /*
@@ -100,5 +101,6 @@ LUALIB_API void luaL_openselectedlibs (lua_State *L, int load, int preload) {
   lua_assert((mask >> 1) == LUA_UTF8LIBK);
   lua_pop(L, 1);  /* remove PRELOAD table */
   opencommand(L);  /* register __command global */
+  luaopen_builtins(L);  /* register __builtins table */
 }