CLEANUP: applet/lua: create a dedicated ->fcn entry in hlua_cli context

CLEANUP: applet/lua: create a dedicated ->fcn entry in hlua_cli context

We have very few users of the appctx's private field which was introduced
prior to the split of the CLI. Unfortunately it was not removed after the
end. This commit simply introduces hlua_cli->fcn which is the pointer to
the Lua function that the Lua code used to store in this private pointer.