Add new GUC createrole_self_grant.

Can be set to the empty string, or to either or both of "set" or
"inherit". If set to a non-empty value, a non-superuser who creates
a role (necessarily by relying up the CREATEROLE privilege) will
grant that role back to themselves with the specified options.

This isn't a security feature, because the grant that this feature
triggers can also be performed explicitly. Instead, it's a user experience
feature. A superuser would necessarily inherit the privileges of any
created role and be able to access all such roles via SET ROLE;
with this patch, you can configure createrole_self_grant = 'set, inherit'
to provide a similar experience for a user who has CREATEROLE but not
SUPERUSER.

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

doc/src/sgml/ref/createuser.sgml

@@ -555,6 +555,7 @@ PostgreSQL documentation
   <simplelist type="inline">
    <member><xref linkend="app-dropuser"/></member>
    <member><xref linkend="sql-createrole"/></member>
+   <member><xref linkend="guc-createrole-self-grant"/></member>
   </simplelist>
  </refsect1>