Make PGOAUTHCAFILE in libpq-oauth work out of debug mode

Hi,

While working on a validator for keycloak[1] with libpq-oauth I find
out that to allow a self-signed certificated I had to set the CA on the
client but for this was required to also set the PGOAUTHDEBUG=UNSAFE
which generated a lot of information on the client side that I didn't
need for my testing and work.

This patch basically remove the need of setting the PGOAUTHDEBUG=UNSAFE
to be able to use PGOAUTHCAFILE.

I'm not sure if where I put the documentation is the right place, I
would like to have some opinions on that matter too.

[1] https://github.com/cloudnative-pg/postgres-keycloak-oauth-validator

--
Jonathan Gonzalez V. <jonathan(dot)abdiel(at)gmail(dot)com>