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

src/interfaces/libpq/fe-trace.c

@@ -40,8 +40,6 @@ PQtrace(PGconn *conn, FILE *debug_port)
 	if (debug_port == NULL)
 		return;
 
-	/* Make the trace stream line-buffered */
-	setvbuf(debug_port, NULL, _IOLBF, 0);
 	conn->Pfdebug = debug_port;
 	conn->traceFlags = 0;
 }

src/test/modules/libpq_pipeline/libpq_pipeline.c

@@ -1319,10 +1319,13 @@ main(int argc, char **argv)
 	/* Set the trace file, if requested */
 	if (tracefile != NULL)
 	{
-		trace = fopen(tracefile, "w+");
-
+		trace = fopen(tracefile, "w");
 		if (trace == NULL)
 			pg_fatal("could not open file \"%s\": %m", tracefile);
+
+		/* Make it line-buffered */
+		setvbuf(trace, NULL, _IOLBF, 0);
+
 		PQtrace(conn, trace);
 		PQtraceSetFlags(conn,
 						PQTRACE_SUPPRESS_TIMESTAMPS | PQTRACE_REGRESS_MODE);