Per buildfarm member crake, any servers including a postgres_fdw server
with this option set would fail to do a pg_upgrade properly as the
option got hidden in f9264d15 by becoming a debug option, making the
restore of the FDW server fail.

This changes back the option in libpq to be visible, but still inactive
to fix this upgrade issue.

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