It's misplaced there -- it's not libpq's output stream to tweak in that
way.  In particular, POSIX says that it has to be called before any
other operation on the file, so if a stream previously used by the
calling application, bad things may happen.

Put setvbuf() in libpq_pipeline for good measure.

Also, reduce fopen(..., "w+") to just fopen(..., "w") in
libpq_pipeline.c.  It's not clear that this fixes anything, but we don't
use w+ anywhere.

Per complaints from Tom Lane.

Discussion: https://postgr.es/m/3337422.1617229905@sss.pgh.pa.us