Restrict the privileges of CREATEROLE users.

Previously, CREATEROLE users were permitted to make nearly arbitrary
changes to roles that they didn't create, with certain exceptions,
particularly superuser roles.  Instead, allow CREATEROLE users to make such
changes to roles for which they possess ADMIN OPTION, and to
grant membership only in roles for which they possess ADMIN OPTION.

When a CREATEROLE user who is not a superuser creates a role, grant
ADMIN OPTION on the newly-created role to the creator, so that they
can administer roles they create or for which they have been given
privileges.

With these changes, CREATEROLE users still have very significant
powers that unprivileged users do not receive: they can alter, rename,
drop, comment on, change the password for, and change security labels
on roles.  However, they can now do these things only for roles for
which they possess appropriate privileges, rather than all
non-superuser roles; moreover, they cannot grant a role such as
pg_execute_server_program unless they themselves possess it.

Patch by me, reviewed by Mark Dilger.

Discussion: https://postgr.es/m/CA+TgmobN59ct+Emmz6ig1Nua2Q-_o=r6DSD98KfU53kctq_kQw@mail.gmail.com

doc/src/sgml/ref/createuser.sgml

@@ -252,8 +252,7 @@ PostgreSQL documentation
       <listitem>
        <para>
         The new user will be allowed to create, alter, drop, comment on,
-        change the security label for, and grant or revoke membership in
-        other roles; that is,
+        change the security label for other roles; that is,
         this user will have <literal>CREATEROLE</literal> privilege.
         See <xref linkend='role-creation' /> for more details about what
         capabilities are conferred by this privilege.