Fix incorrect permissions on pg_subscription.

Fix incorrect permissions on pg_subscription.

The documented intent is for all columns except subconninfo to be
publicly readable.  However, this has been overlooked twice.
subsynccommit has never been readable since it was introduced,
nor has the oid column (which is important for joining).

Given the lack of previous complaints, it's not clear that it's
worth doing anything about this in the back branches.  But there's
still time to fix it inexpensively for v14.

Per report from Israel Barth (via Euler Taveira).

Patch by Euler Taveira, possibly-vain comment updates by me.

Discussion: https://postgr.es/m/b8f7c17c-0041-46b6-acfe-2d1f5a985ab4@www.fastmail.com