Allowing frontends to use concurrent-safe open() and fopen() via 0ba06e0b
has the side-effect of switching the default translation mode from text
to binary, so the switch can cause breakages for frontend tools when the
caller of those new versions specifies neither binary and text.  This
commit makes sure to maintain strict compatibility with past versions,
so as no frontends should see a difference when upgrading.

Author: Laurenz Albe
Reviewed-by: Michael Paquier, Tom Lane
Discussion: https://postgr.es/m/20180917140202.GF31460@paquier.xyz