This partially reverts 096bbf7c and 9d2d4570, undoing the libpq changes as
it could cause breakages in distributions that share one single libpq
version across multiple major versions of Postgres for extensions and
applications linking to that.

Note that the backend is unchanged here, and it still disables SSL
compression while simplifying the underlying catalogs that tracked if
compression was enabled or not for a SSL connection.

Per discussion with Tom Lane and Daniel Gustafsson.

Discussion: https://postgr.es/m/YEbq15JKJwIX+S6m@paquier.xyz