psql: Sprinkle more CppAsString2() in describe.c

Like 91f5a4a for pg_amcheck, this makes the code more
self-documented as there is less need to look in the headers what a
hardcoded value means.  This touches queries related to procedures, AMs,
functions, databases, relations, constraints, collations, types and
extended stats, pulling into psql their *_d.h headers.  The queries are
written the same way as originally.

There are still a couple of hardcoded values.  These cannot be included
yet as they are not exposed in headers that are safe to use in frontend
code.

Note that describe.c was including pg_am.h that should be used only in
backend code.  This is updated to use pg_am_d.h.

Reviewed-by: Daniel Gustafsson, Corey Huinker
Discussion: https://postgr.es/m/Zxb2hpca-pZc6zKe@paquier.xyz

Author: michaelpq