Add macros in hash and btree AMs to get the special area of their pages

Add macros in hash and btree AMs to get the special area of their pages

This makes the code more consistent with SpGiST, GiST and GIN, that
already use this style, and the idea is to make easier the introduction
of more sanity checks for each of these AM-specific macros.  BRIN uses a
different set of macros to get a page's type and flags, so it has no
need for something similar.

Author: Matthias van de Meent
Discussion: https://postgr.es/m/CAEze2WjE3+tGO9Fs9+iZMU+z6mMZKo54W1Zt98WKqbEUHbHOBg@mail.gmail.com