Add new catalog called pg_init_privs

This new catalog holds the privileges which the system was
initialized with at initdb time, along with any permissions set
by extensions at CREATE EXTENSION time.  This allows pg_dump
(and any other similar use-cases) to detect when the privileges
set on initdb-created or extension-created objects have been
changed from what they were set to at initdb/extension-creation
time and handle those changes appropriately.

Reviews by Alexander Korotkov, Jose Luis Tallon