Simplify proof of auth_local_update.

Also, use explicit unfolding lemmas for auth_valid and auth_validN.
The `Arguments valid _ _ !_ /` hack did not really work when one
has to deal with the valid instance of the cmra, which underneath also
includes a `cmra_valid`. Declaring a similar Arguments for `cmra_valid`
is a bad idea, it will also end up unfold stuff for the exclusive and
option CMRA.