ubus_rpc_session is not ignored when checking policy

[wryun]

If you develop a ucode rpcd plugin, when you specify the possible args (i.e. the policy) you can leave out ubus_rpc_session due to:

https://github.com/openwrt/rpcd/blob/d4fa147470aab6aee4b23ef6327201c6be069ab1/ucode.c#L309

However, if you create a daemon and use ubus.publish, there is no equivalent code in lib/ubus.c, so you have to specify that it allows ubus_rpc_session manually if you want it to be called via uhttpd.

It would be good if the handling between these two were consistent. We ran across this when converting code from a plugin to a daemon, and the new 'works in backend, but frontend gives INVALID_ARGUMENT' behaviour was confusing.