pgsql: Remove setvbuf() call from PQtrace()

Remove setvbuf() call from PQtrace()

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6ec578e60101c3c02533f99715945a0400fb3286

Modified Files
--------------
src/interfaces/libpq/fe-trace.c                  | 2 --
src/test/modules/libpq_pipeline/libpq_pipeline.c | 7 +++++--
2 files changed, 5 insertions(+), 4 deletions(-)