Tying an object's ownership to datdba

https://postgr.es/m/20201031163518.GB4039133@rfd.leadboat.com gave $SUBJECT as
one of the constituent projects for changing the public schema default ACL.
This feature stands on its own, hence the new thread.  I showed syntax "ALTER
SCHEMA public OWNER TO DATABASE_OWNER" and anticipated a new RoleSpecType.
That was fine for in-memory representation, but it lacked a value to store in
pg_namespace.nspowner.  That led me to instead add a default role,
"pg_database_owner".  That way, user queries joining owner columns to
pg_authid need no modification.  Also, user.c changes were trivial, and
pg_dump needed no changes.  The role's membership consists, implicitly, of the
current database owner.  The first patch refactors acl.c to reduce code
duplication, and the second patch adds the feature.

I ended up blocking DDL that creates role memberships involving the new role;
see reasons in user.c comments.  Lifting those restrictions looked feasible,
but it was inessential to the mission, and avoiding unintended consequences
would have been tricky.  Views "information_schema.enabled_roles" and
"information_schema.applicable_roles" list any implicit membership in
pg_database_owner, but pg_catalog.pg_group and psql \dgS do not.  If this
leads any reviewer to look closely at applicable_roles, note that its behavior
doesn't match its documentation
(https://postgr.es/m/flat/20060728170615.GY20016@kenobi.snowman.net).

This patch makes us read pg_database when reading pg_auth_members.
IndexScanOK() has been saying "during backend startup we have to be able to
use the pg_authid and pg_auth_members syscaches for authentication".  While
that's true of pg_authid, I found no sign of pg_auth_members reads that early.
(The read in InitPostgres() -> CheckMyDatabase() -> pg_database_aclcheck()
happens after RelationCacheInitializePhase3().  In a physical walsender, which
never has a mature relcache, some SHOW commands make guc.c check
DEFAULT_ROLE_READ_ALL_SETTINGS.  The walsender case, though it happens after
authentication, may necessitate IndexScanOK()'s treatment of pg_auth_members.)
For now, just in case I missed the early read, I've made IndexScanOK() treat
pg_database like pg_auth_members.

Thanks,
nm