Also, higher cost for [elim_modal_bupd_fupd], so that it is not taken in place of [elim_modal_fupd_fupd] in spec patterns.

When having H : ▷ (P -∗ Q) and H2 : ▷ P, iSpecialize ("H" with "H2")
distributes the later over the wand.

The rewrite auth_validN_eq was not performed in the hypothesis. It
used to work in 8.5 because of magic.

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.

This fixes issue #46.

Proof was done by Hai & me

This fixes issue #44.